Corrected sentence: Moray eels hide in coral and grab any small fish that swims by.
Your sentence was mostly good. I made a few small changes and I will explain them simply:
- coral vs. corals: We usually say "coral" when talking about the reef as a place to hide. Saying "in coral" or "in the coral" sounds natural.
- little vs. small: Both words work, but "small" sounds a bit more natural here. "Little" is okay too if you like it.
- swims vs. swim: You used "any small fish that swims by." That is correct because "any" talks about one fish at a time, so we use "swims." If you were thinking of many fish at once, you could say "swim."
- Capitalization: Starting the sentence with "Moray" is fine because it is the first word.
Other good ways to say the same thing:
- Moray eels hide among coral and grab any small fish that swims by.
- Moray eels hide in the coral and grab any little fish that swims by.
Nice job — your sentence was clear! Keep practicing writing sentences like this.