I meant to indicate I now know what the species is (in addition to the common name you provided in post #9 and my acknowledgment in post #15 that what I have is the same or similar to what you have).
After finding out the species I read on some other sites that they're not necessarily photosynthetic. I'm not sure if that applies to some or all species. Do you feed yours regularly? I'm feeding it krill about twice a week as well as provide enough light over it to sustain common mushrooms.
It surprised me a week or so ago when I caught it with its mouth gaped fully open expelling a fairly large (and unfortunately, digested) nasarius snail and as well as a lot of sand; it must have caught it a night or two before, I totally forgot I had that snail in there).