The short answer is sometimes they are, and sometimes they are not...What size is your tank, what are your salinity levels as well as ammonia, nitrite and nitrate levels? What are you using to measure the salinity as well.
Thanks for the mension fanatic, but ive only been at the saltwater hoby a few months myself and have a small tank but my advice for the op because he has had tanks for years would be to think if he has been doing/using anything different atall even simple things such as changing an adative/salts brand and if that's the case look deep into products, because I couldnt comment on the urchin or crab I know nothing of them in the slightest. Also think has anything like a nem or corals died ? Theres not a lot of info to work with.
I had a problem with an emerald crab going after my fish and cleaner shrimp after lights out. My lights are on a timer so I set them to blue to watch and caught him red-handed. He went straight into the refugium.