I could probably tell you for sure what it is if you can post a good picture of it on here. In all of my experience, sea hares arent very hardy creatures, and with all creatures, acclimation is the hardiest part. The fact that it hasnt moved since going into your tank isnt really a good sign.
How did you acclimate? Drip? Float? How long?
A tell tale sign that a sea hare (or nudibranchs for that matter) is going to die, is if it lays eggs in your tank. Sea hares usually lay long yellow ribbon like eggs.
Did you notice these yellow marks at the LFS before you bought him?