Was Najashi A muslim ?

In the name of Allah most gracious most merciful
Assalaamualaikum wa rahmatuallahi wa barakatuhu

Najashi [al-Negus, the Ethiopian ruler] was a christian who later accepted Islam

Sahih Al-Bukhari HadithHadith 2.337 Narrated byAbu Huraira
Allah's Apostle informed (the people) about the death of An-Najashi on the very day he died. He went towards the Musalla (praying place) and the people stood behind him in rows. He said four Takbirs ( i.e. offered the Funeral prayer).

"And there are, certainly, Among the People of the Book, Those who believe in God, In the revelation to you, And in the revelation to them, Bowing in humility to God: They will not sell The Signs of God For a miserable gain! For them is a reward With their Lord, And God is swift in account." S. 3:199

"Jabir b. Abdallah, Anas, Ubn Abbas, and Qatada said: 'This verse was inspired concerning the death of al-Negus, the Ethiopian ruler. Gabriel announced his death to Muhammad on the day in which he died. Muhammad said to his followers: "Get out and pray for a brother of yours who died in
another land." They asked him: "And who is he?" Muhammad answered: "He is Negus." Then Muhammad prayed "Allahu akbar" four times and asked forgiveness for al-Negus. He also said to his followers: "Ask forgiveness for him." So the hypocrites [Muhammad's enemies] said: " Look at that man! He prays for a Christian Ethiopian infidel whom he had never seen before and who does not
follow his religion." Muhammad said: "Of the people of Scriptures." 3:199 Mujahid, Ibn Juraij, and Ibn Zaid said: "This verse was inspired to describe the entire people of the Book"' (see al-Qurtubi's commentary on Sura Al Imran 3:199)"

Muslims are not supposed to pray for non-muslims[kuffar]
"It is not for the Prophet, and those who believe, to pray for the forgiveness of idolaters mushrikiina) even though they may be near of kin (to them) after it hath become clear that they are people of hell-fire." S. 9:113

Najashi's aceptace Of Islam:
Ishaq also reports that Muhammad sent a letter to the Negus, inviting the latter to embrace Islam:

"The apostle sent Shuja' b. Wahb, brother of B. Asad b. Khuzayma, to al-Mundhir b. al-Harith b. Abu Shimr al-Ghassani, lord of Damascus... (T. via Salama. The apostle sent 'Amr b. Umayya al-Damri to the Negus about Ja'far b. Abu Talib and his companions and sent a letter with him ... 'From Muhammad the apostle of God to the Negus al-Asham king of Abyssinia, Peace. I praise Allah unto you the King, the Holy, the Peace, the Faithful, the Watcher, and I bear witness that Jesus son of Mary is the spirit of God and His word which he cast to Mary the Virgin, the good, the pure, so that she conceived Jesus. God created him from His spirit and His breathing as He created Adam by His hand and His breathing. I call you to God the Unique without partner and to His obedience, and to follow me and to believe in
that which came to me, for I am the apostle of God. I have sent to you my nephew Ja'far with a number of Muslims, and when they come to you entertain them without haughtiness, for I invite you and your armies to God. I have accomplished (my work) and my admonitions, so receive my advice. Peace upon all those that follow the true guidance.'

"The Negus replied: ... 'From Negus al-Asham b. Abjar, Peace upon you, O prophet of Allah, and mercy and blessing from Allah beside Whom there is no God, who has guided me to Islam. I have received your letter in which you mention the matter of Jesus and by the Lord of heaven and earth he is not
one strap more than what you say. We know that with which you were sent to us and we have entertained your nephew and his companions . I testify that you are God's apostle, true and confirming (those before you). I have given my fealty to you and to your nephew and I have surrendered myself through him to the Lord of the worlds. I have sent to you my son Arha. I have
control only over myself and if you wish me to come to you, O apostle of God, I will do so. I bear witness that what you say is true. '

"I was told that the Negus sent his son with sixty Abyssinians by boat, and when they were in the middle of the sea the boat foundered and they all perished.)" (Ishaq, pp. 657-658)

Allah knows best.