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?
-
It seems that Meerus has a great reason for hiding his true power. Whatever that reason is, doesn't matter at this point. Perhaps he doesn't want high levels (like Beerus / Supreme Kai / etc) finding out about him. Since allegedly Beerus / Supreme Kai searched their universe for the strongest fighters during TOP and concluded that Goku was the best (aside from the Monaku joke), but Meerus is actually better than Goku, means that they do not know Meerus exists or is as powerful as he really is.– Eric FCommented Nov 8, 2019 at 20:40
1 Answer
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.