Sea slug?? or something?

I also saw one of these at Lovers Key, it looked dark purple almost black in color and felt like a slug or snail, but when I reached out to touch it, it released a purple dye. I can't find a match either, at first I thought it was a leaf of some kind but then I saw it swim to the bottom and back up again very cool to watch.. it seemed fascinated by my friends and I.