The body shape is all wrong for a Pycnogonida (sea spider) without a better picture I'm almost hestitant to guess. I'd like to see the whole animal because I feel there are parts not shown that will reveal the clue to what this animal truly is.
I'm with David on this one. Definitely not a sea spider, but very unsure what it actually is without a better pic. My money is on some type of cnidarian though.
Feed it. If it doesn't grab a small piece of seafood and quickly collapse around it then it's not Aiptasia. Sure looks like Aiptasia to me in the pic though.
ok guys this thing is in my tank but a friend posted it for me and when i move the rock it came in i get to see it seems to like it there but it moves like a octopuse. there was two of them and my clown did try to eat one and spit it out i guess it wasnt that tasty hehehe.
Pull it out of the tank and put in a container with a bit of tank water so you can get a clearer pic of it. Take a few shots of different angles, top, side, underside etc. TY!