Help with "Crab" ID

Hi all.
I need help ID'ing something that looks liike a crab in my 9 Mo. old 10 gal
nano-reef. I woke up this morning and before turning any lights on inside
the tank just happened to look in and see a small white tarantula looking
creature near my ricordia. It is about 3/4" and I couldn't really tell if it
was a crab or a starfish. It moves like a crab and the ends of the legs are
pointed like my other crabs. I never bought it but it has probably been
there for months, any idea what it could be/eat and if its dangerous?
Digital camera doesn't seem to be an option as I have never seen it and
probably wont be able to find it readily. Links to sites with info would be
great too!!!

Thanks
Some guy with fish.