I am not sure what kind os sea cucumber you are referring to. However, Sea cucumbers are usually releases toxin when injured or under excessive stress. You should take the animal out and place it in a different tank, if you do not have a different tank, then you need to decide if you want to keep the animal or you want to keep your tank.
You need to change your water. 1/3 of the total tank water for at least three days. After the initial water change, you should also change your carbon. Let ehe skimmer continue to run, OK???
HTH!
