+1 on the aiptasia....seems to be it! removing that's gonna be tough unless you can get to the rock, throw some putty over it and let it cure or coral glue...
So everyday I am finding at least 2. I've noticed the bigger they are the easier it is to kill them. I just killed 2 big ones not sure if this small one I found is Aptasia or a feather duster. It's tiny so I juiced em!!!