Poor little guy was just hanging out by one of his caves yesterday when the green brittle star "goosed" him and he turned and smashed the lower left side of his face against the glass...HARD.
He now has a bruise/blood on/in his gill plate and chin. he is still eating as of this afternoon and relatively active, but I can tell he's not completely himself.
Sadly I know there is little that can be done but wait it out.
I lost an Orbic bat once from the very same thing. Another fish barely nipped his tail and he turned and slammed into the glass so hard he popped his eye.
Happened right in front of me.
He passed the next day. I raised him from dime sized up to about the size of a plate, excluding fins.
I sure hope my little guy will be ok.