Yup, that is where it poops from
Urchins losing all their spines and sitting in the same place are not likely to recover. Anything is possible, but it is not a good sign. It is normal for them to lose or break spines here and there...and those grow back, but I suspect this one is not doing well. It might be dead. Since there is little soft tissue on the outside, you may not notice much when these things die except that the spines start to fall off. If you look real close, do you see and little thread like tube feet waving around?
Since it was a relatively recent acquisition, it might have suffered some acclimation shock. Individuals can have different tolerances.