My algae eater doesn't eat algae

I never see my algae eater move. He lives in a rock and he's very
alive... and he must be eating but I never see him on the side of the
tank or doing anything even close to eating algae. Honestly I can't
tell if the algae is getting eaten or not, I never see him out of the
rock. I chase him out when I do a water change and I've seen him poop
so he's eating something. Do some algae eaters not eat algae? Or is
he just sneaking out of his rock in the middle of the night when I'm
not looking?

Laura