They are very easy to harm if they get sucked into water intakes or pumps. Which happens pretty easily. So hopefully you have a blocked off water intake. They are similar but still pretty far from nudibranchs. I would be concerned about if they release a toxin if they get stressed or die, because I do not know if they do or don't. They move rather quickly and actually are pretty good at controlling some of the harder to manage algae in a system. They have ways of chewing it off of rocks because it has "teeth". They are also relatively easy to care for, but still do not tolerate certain things like many other inverts out there, nitrates, nitrites, etc.