So in Dragon Ball Super manga #51 shows that Moro is stronger than a Goku SSJ3. And later in manga #52 he goes with Goku to the time chamber to train, having Goku the intention to train with him to be able to awaken Ultra Instinct again. So if Merus is strong enough as for being a sparring of Goku, why didnt he join Goku, Vegeta and Buu to fight Moro?
Chapter 55 explained that Merus was an angel-in-training, whose laws forbid them from fighting.
As explained by the Grand Minister and Whis, angels are ruled by several laws designed to prevent undue intervention within mortal events. They are to stay as neutral as possible, and are not allowed to use their angel powers as long as they are in training.
Merus acting as a Galactic Patrol agent was allowed as a "grey-area" type of behavior, since he only used standard-issue guns, armors and gadgets to complete his missions (along with his natural fast speed). Likewise, Merus training Goku was allowed as he didn't use his angelic powers and only imparted knowledge upon a mortal.
You can read about these laws and restraints in Dragon Ball Super 55's official release on Mangaplus.