james, it sounds like it is a aiptasia spp. aka triffid, glass or rock anemone but me not too sure but the way u discribe it sure sounds like it.. if so, they are basically wolfs in sheeps clothings.. they are nice to look and interesting but will multiply to very unsightly proportion..
their sting is capable of killing a small fish and bigger fishes like your puffer fish might get a nasty sting out of it..
if your tank is indeed invaded by such aliens, options will be to scrub the rock under hot running water (but risk having the LR dying off), introduce fishes that eats them like Large angelfishes or butterfly fishes(which i dun recommend) or kill them off with copper-based medication(which i also dun recommend)...
Let us work together to preserve the world for our children to inherit by being responsible to our surroundings. Take nothing but pictures, leave nothing but footprints, bubbles and memories.
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