Diamond Goby missing half of his lower lip

[Jeter22]

My Diamond Goby hid all day yesterday and didn't come out to eat. Today he is out and eating the food I am feeding the tank. I noticed that his lower lip is half gone. Looks like it has been rubbed raw. I do have a Redlegged Hermit Crab. The Hermit Crab and the Diamond Goby have lived together for 3 months.
Can anyone tell me what could have happened to the Diamond Goby's lip? I've attached a picture.

 

[carey]

It looks like you have large gravel, maybe it's from sifting that? They usually have a finer sand to deal with so that might be too big and sharp for him.