whats this?

[whipit]

 

[olemiss reb]

nudibranch, I'd get rid of it.

 

[LeslieH]

More to the point, it's a nudi that specializes in eating zoanthids and increases rapidly in population size. Get it out, do a visual search of your zoas for more adults & egg ribbons, and start dipping. There must be dozens if not hundreds of threads here about how to deal with these which will be useful for you.