What is it, please?

Is this some sort of undersea anemone? Very brilliant green.

I think that you are correct Greenthumb, but usually the anemones have finger like appendages that stick out and collect food. I'll do some research and get back to you GOYs :)

Anemone can retract the fingers when startled or not feeding. Like a venus fly trap type movement.

Still havn't found a photo confirming that this is an anemone, but I think that Greenthumb is right.