OK, might be able to help somone here for once! I had a few sea cucumbers and has a similar thing happen to me. Found out later that when they get distressed they can expel most of their internal organs. The long stringy thing is part of the intestines I believe. Someone here told me that they can regenerate if the tank is running very sweet, but something stressed it in the first place, so I wouldn't count on it. Have also come to the conclusion the the risk of having a cuke die and release all those stored toxins into my reef tank wasn't worth it, so they came out. I don't think it should be that much of a worry in a FO tank. Do a search here on sea cucumbers and you should find the message where someone gave me a much better descrition of what is happening to you. Hope that helped.