I have 6 in my 100 gallon.
As others have said, they are usually found on the sandbed. Top layer, eyes out, slowly moving and eating. I have one or two that will climb on the glass and rocks. However, they do not party there too long. I think they fall over more readily by climbing the rocks. So - no no.
The primary food I feel is algae. Mostly diatoms and red slime / cyno. How good of a job at eating cyno, I do not know. But, I see them eating it.
With only 6 in the tank I do not think they are controlling it. But, they do seem to keep it in check. In fact, I think they are perfect for straffing the top layer of sand without hurting your fauna.
I'd recommend them.