Sea Urching dying?

[DACJ]

I have 3 sea urchins in my 29G biocube, all different in size. Recently the biggest one has lost most of its spikes, and often fall of the sand bed everyday if not more than once a day. They other two of the smaller sized sea urchins and all my fish are still doing fine so I don't suspect it to be a watch quality problem...

Does anyone have any ideas on whether anything may be happening to it specifically? Or is it just dying cause of aging?

Now, I know people often suggest putting it in the freezer to euthanize the urchin, however I have not done that because it still eats, just hard to get around and I AM hoping that it will come back to normal state....

More over, it has fallen into the centre of the tank where I cannot reach it because of all the LR settings....I can see half of it's body, I still it little movement of tentacles and small spikes, but it's been in the same place for at least 8 hrs now, IF it does die, should I remove all the settings and remove the urchin? or can I leave it in there to be dissolved or whatsoever?

Or perhaps someone can give me insights on that it is not dying?

Thanks a lot!