I believe they are Warty Jellyfish
http://www.sanpedrosun.net/old/07-153.html and they will get your attention. It took me a while to realize that the ones with short (<1 inch) purple tentacles and the ones with long tentacles (>1 meter) were the same just with tentacles withdrawn. Most dives had few to none. But on some dives they were numerous and concentrated in the rest stop zone. I was amusing myself at one reststop tapping them on the dome to make them withdraw their tentacles when I saw some long slender tentacles inches in front of me. The initial sting is startling, but it quickly subsides to tolerable and the red welts disappear in a few hours. Those that drift too close to the top of the reef get caught in the gorgonians which appear to be eating them. So if you are feeling lucky, you could always try feeding the gorgonians.