The incident of Banu Mustaliq took place as a consequence of this tribe planning to attack the Muslim community. The historical evidences tell us that the Banu al-Mustaliq tribe did plot to attack and kill Muslims. Hence, with news reaching the Prophet (p) on this, he had to act quickly.
Historical reports on this incident
“THE RAID ON B. AL-MUSTALIQ
The apostle stayed in medina during the latter part of Jumada’l-Akhira and Rajab; then he attacked b. al-Mustaliq of Khuza’a in Sha’ban A.H. 6.
Asim b, Umar b. Qatada and Abdullah b. Abu Bakr and Muhammad b. Yahya b. Habban each told me a part of the following story:
The apostle received news that B. al-Mustaliq were gathering together against him, their leader being al-Harith b. Abu Dirar, the father of Juwayriya d. al-Harith (afterwards) wife of the apostle. When the apostle heard about them he went out and met them at a watering place of theirs called al-Muraysi in the direction of Qudayd towards the shore. There was a fight and God put the B. al-Mustaliq to flight and killed some of them…” 
“EXPEDITION AGAINST THE BANU AL-MUSTALIQ
… They said: Word reached the Messenger of God that the Banu al-Mustaliq were gathering against him under the leadership of harith b. Abi Dirar. … When the Messenger of God heard about them, he set out toward them and met them at one of their watering places called al-Muraysi, near Qudayd toward the coast. The people advanced toward each other and fought fiercely. God put the Banu al-Mustaliq to flight and killed some of them. …” 
Kitab al-tabaqat al-Kabir – Ibn Sa’d:
“GHAZWAH OF THE APOSTLE OF ALLAH, TO AL-MURAYSI
Then (occurred) the ghazwah of the Apostle of Allah, to al-Muraysi in Sha’ban in the fifth year from his Hijrah. They (narrators) said: Verily, Banu al-Mustaliq were a branch of the Khuza’ah, and were allies of Banu Mudlij. They had halted at a well, known as al-Muraysi which lies at a day’s (journey) from al-Fur. ) The distance) between al-Fur and al-Madinah is eight burds (96 miles).
Their chief and leader was al-Harith Ibn Abi Dirar. He moved among his people and those of the Arabs whom he could approach, and exhorted them to fight against the Apostle of Allah. They responded and prepared themselves for marching with him. (The news) reached the Apostle of Allah, who sent Burayday Ibn al-Husayb al-Aslami to collect information about this. He went there and met al-Harith Ibn Abi Dirar and had a conversation with him.
He returned to the Apostle of Allah, and furnished him with the information. Thereupon the Apostle of Allah, called the people and they immediately set out. They had thirty horses, ten with the Muhajirs and twenty with the Ansars. Many of the hypocrites, who had never accompanied in any Ghazwah, joined him (Prophet). He appointed Zayd Ibn Harithah as his vicegerent. He had two horses Lizaz and al-Zarib with him. He set out on Monday, 22 Sha’ban. …” 
Scholars commenting on this incident
Shaykh Allamah Shibli Nomani:
“GHAZWA MURISI OR BANU MUSTALIQ (5TH HIJRA SHA’BAN I.E., JANUARY, 627 C.E.)
Khuza’a was the tribe in alliance with the Quraish. Once it struck the Quraish that, as descendants of Ibrahim (p) they must in every way enjoy distinction from others. A short stay in the open plain of Arafat is a part of Hajj ceremonial. This open space was situated outside the boundary of Haram.
The Quraish made it a rule that, while others should go to Arafat, they would stay at Muzdalifa, a place within the boundary of the Haram. Appropriating to themselves similar other privileges, they had assumed the title of ‘Ahmas’. Nevertheless, they had been kind enough to allow this appellation to others provided they submitted to their restrictions, and with such people they condescended to have matrimonial ties. The tribe of Khuza’a was one of those thus honoured. The Banu Mustaliq, a branch of the Kuza’a, inhabited the place known as Murisi, which was nine marches away from Medina. Harith Ibn Dirar, the chief of the tribe, acting on his own initiative or at the instigation of the Quraish, made preparations for an attack on Medina.
On receipt of information the Holy Prophet (p) sent Zaid Ibn Khusaib for further enquiry. Zaid, on his return confirmed the report, and the Prophet (p) ordered his companions to get prepared for a battle. On the second day of Sha’ban, the Muslim army set out from Medina. When the news reached the people at Murisi, the fighting force under Harith melted away, and he himself managed to escape. But the inhabitants of Murisi put themselves in battle array and kept up a steady downpour of arrows for a long time. The Muslims made a sudden all-out attack which put the opponents to flight. Ten of their men were killed and the rest made prisoners. They numbered 600. Two thousand camels and 5,000 goats were captured as booty.” 
Muhammad Husayn Haykal:
“The news reached Muhammad that the Banu al-Mustaliq, a clan of Khuza’ah tribe, were mobilizing for war in the vicinity of Makkah and inciting the Arab tribes around them to assassinate Muhammad. Their leader was al-Harith Ibn Abu Dirar. Acting quickly in seizing the initiative, Muhammed hastened to strike and take them by surprise as was his custim. The two divisions of Al-Muhajirun and al-Ansar which rallied to his immediate call were led by Abu Bakr and Sa’d ibn Ubadah, respectively. The Muslims were encampled near a well called al-Muraysi, not far from the encampment of their enemies. The allies of Banu Mustaliq ran away upon hearing the news of the advancing Muslim army, with the result that the Banu al-Mustaliq themselves were quickly encircled. In the short engagement which followed, the Muslims lost one man, Hisham Ibn al-Khattab by name, who was killed accidently by a fellow Muslim. After losing ten men, the Banu al-Mustaliq realized that they had better surrender to the Muslims forces. …” 
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 Ibn Hisham, [Translator: Alfred Guillaume]. The life of Muhammad: a translation of Isḥaq’s Sirat rasul Allah, page 490
 History of al-Tabari: The: The Victory of Islam: Muhammad at Medina – volume 8, page 51
 Kitab Al-tabaqat Al-Kabir, by Ibn Sa’d, volume 2, page 77
 Sirat -un- Nabi [Sallallahu Alaihi Wasallam] By Shaykh Allamah Shibli Nomani (r.a) – volume 2, page 100 – 101
 The Life of Muhammad [Isamic book Trust 2008] By Muhammad Husayn Haykal, page 352