Of his prowess, the romance says that Sir Morien's "blows were so mighty; did a spear fly towards him, to harm him, it troubled him no whit, but he smote it in twain as if it were a reed; naught might endure before him."
After Morien tells his story to Lancelot and Gawain, who promise to help him find his father, the knights go on a series of adventures showcasing their talents.
Within the Dutch Lancelot text, Morien mentions brief moments of racism like no one wanting to take him across the water and Sir Gariët being afraid of him at their first encounter.
While they do end up joining forces to find Morien's father and resolving their initial apprehensions, it is clear that a character "black of face and limb" was an uncomfortable concept for writers at the time.
However, it is also important to note that Gawain and Lancelot do not see Morien's face until after the battle is complete, and make the judgement based on his armor, rather than his physical appearance.