Black Rock Urchin losing its spines



Hello! I am new to the saltwater tank business. I have a 55 gallon, fish only tank, with a 6 gallon canister filter. Ive had the tank up and running for 2 and a half months and recently added fish 2 weeks ago. I bought 2 black rock urchins and today I noticed one of them is losing its spines. The other one looks perfectly fine and I also have a brittle star that seems to be doing fine as well. I haven't seen this urchin eat anything, it just stayed in one spot after day 2 (yesterday) and today it hadent moved at all and even lost quite a few spines. It looked completely fine in the store, and like I said my other urchin looks good and the brittle star looks good too. The other urchin doesn't move much, but it moves a little each day, staying in the same area. I moved the urchin that is losing its spines on the rocks where I know there is bacteria/algal growth and I put a small bit of seaweed right by its mouth. It is still alive, but barely moving. All my water parameters are normal and within what the urchins can tolerate, the salinity is perfect as well. I am very meticulous about testing the water. Any help would be appreciated!
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