Argh need a coral doc.. im stumped

[ret25yo]

Gone for a week came back to this..only thing added before was a pithos crab and another turbo snail. Been in tank 6 months.. occasionally sanded by diamond goby.. any ideas what getting at it?

 

[blaxsun]

My first thought is the goby dive-bombing it with sand (as both spots are covered in sand). Just a guess, though. I don't think it's the turbo or the crab (unless the crab was cleaning out some algae growth).