Unless it is in an awkward pose.....Arrows (Stenorhynchus sp.) have their legs relatively uniformly radially oriented. In the pic, rear legs are more laterally oriented. I would lean toward one of the spider decorators (pycnogonid sp. ++ thousands of species) See Krazykarl's WTF thread this page (one of his earlier cases of crabs... ) for a shorter limb member of the group. Not as nasty a predator as Arrows become but an opportunistic carnivore all the same if not kept well fed....
Yeas i was leaning toward a spider crab. Probascus something ...
Anyway i got him free in a bag n he was alot smaller but still pink.
I dont have pink sponge anywear in my tank. Just blue n beige..