Something eating Acropora

So I got back from a trip this past weekend and was checking out my tank when I noticed something had bit off one of the tips on my acropora frag (i.e. took some skeleton with it). I had noticed this once before but I had written it off as damage from a "rock slide" I'd inadvertently cause while siphoning the tank the day before. This time there's no way anything struck it.



Current Mobile tank inhabitants:

Royal Gramma

Cherub Dwarf Angelfish

2 rock urchins

Snails (astrea, nassarius)

Tiger shrimp

pistol shrimp of unknown variety (hitch-hiker)



Other issues that may be ralated:

platygra brain coral had some unexplained damage that has since healed

1-2 years ado had a series of unexplained fish disappearances (never found the culprit)



Questions:

1. How do I go about find out what's causing this?

2. What are likely suspects here? (current thoughts: Eunice worm, xanithid crab)



Thanks in advance