This is a giant carpet sea anemone(Stichodactyla gigantea). When fishes go within touching range near them,the anemone's tentacles will shoot out toxic spines(similiar to a jellyfish)and the tentacle will stick to the fish,injecting toxins into the fish.Then,the tentacles will drag the fish directly to the center of the anemone,where the mouth is located.Sometimes,anemone shrimps or clownfishes(the most famous clownfish in the 21st century,Nemo) are found here.The anemone protects them,the little critters bring food to it.So far,we didn't see any anemone shrimps or clownfishes in there.Probably off to look for food,or they didn't make it back.
http://aquaticwhispers.blogspot.com/...t-of-june.html
any chances that you touch your mouth or an open wound? I've only experienced something similar with xenia but no stiffness. Just the smell that made me sick and wanted to vomit.