Advice on cucumbers?

So, in this new tank we also have a cucumber. 90g reef, if you haven't seen my other posts yet. I think it's a Spiny Sea Cucumber. It's only 2-3 inches at rest it seems, but we saw it stretch out a bit once and it looked longer than that. I thought these guys lived to sift sand? So far I've only seen it cling to the clam for about 6 days. I've begun to do some reading on these things, and I'm beginning to wonder why anyone would keep one in an aquarium. I had no idea they were as dangerous to keep as what I'm reading is leading me to believe. I love this tank, and the cucumber is cool and all, but am I taking some huge unnecessary risk by having this thing in the tank? Could it really kill everything in the tank without warning?