I hate these things. They give me the creeps. In the olden days, we killed them if we saw them-considred them a nuisance rather than a benefit. Ugly and they do eat-or bore into rocks and stony corals. you will see their enlarged holes filled with rock dust. Yours isnt in the sand-mine never is either-they are in the rocks because they like them-and I dont know about you, but I dont have any detritis or anything on my rock, but good stuff.
If he is a beneficial scavenger, then he should be down there in the 'dirt' scavenging, not makin holes in my rocks and maybe, just maybe the corals. I have one just like yours and I am camping out with a flashlight trying to snag him. He has ignored my worm trap and I think he is on to me because he stays real close to a hole to jump into when he sees me coming!