Any idea?

[Emmanuel Zamora]

Looks like a little worm. Sorry I know it’s small

 

[tdileo]

Looks like a ghost flatworm to me. Not a problem.

 

[KJ]

Brown flatworms.

 

[Billdogg]

The only problem is that it's population can explode almost overnight. If there's only a few, I would suggest manual removal. If it's more than a few, many wrasses love to snack on them. I have a pair of Blue Star Leopard wrasses that eliminated a fairly heavy load of them in just a week or two. As an added bonus, IMHO one of the more stunning reef safe fishes around.