Sea Creature, Talisker Bay

I am not sure what the creature is. Some friends and a local librarian thought it was a sea-slug, but I think it is more in the jelly-fish class. It had attached itself to a rock and there were others floating in rock-pools.  When they’re floating they are inverted, with little stubby tentacles round the perimeter. The transparent membrane attaching it to the rock becomes a sail.

Clever.