Yup, kinda sounds like the feelers. Does the threadlike thing dissolve eventually (like a mucus string) or stay with the frogspawn tentacle? If the latter, it's the feelers. Ours did that and stung a piece of pumping xenia which then crashed. Make sure you give the frogspawn lots of room. Our feelers don't seem to come out often, but we learned and have given it a zone of its own.
Try target feeding the frogspawn - maybe he's hungry. We put a small/teeny piece of shrimp on a skewer and lower the skewer into position so that the shrimp is up against a frogspawn mouth (little puckered bumps at the top of the stem area). The spawn tentacles then curl around the shrimp piece and we pull off the skewer and the tentacles push the shrimp into the mouth. Isn't it amazing all the stuff that fish and other critters teach us to do
