Mine did it yesterday. I don't recall it being spooked for any reason but who knows, I wasn't really watching. My fiancee went to feed him or put a piece of seaweed on the clip or something and asked me what that goo was.
Mine loves seaweed and plays with the empty clip. I think that they are playful, I see mine play in the powerhead current and trying to hang out with my lionfish all the time. A great addition imo.