sea hares are a close relitive to the nutisbriks and yes the back should open. if it is opening it's back it is because of 1 of 2 things it is happy or it is scared. It normaly takes 5 hrs to aclimate them so he may have been a bit tick off at you it will finish aclimatizing in the tank just will take longer. watch it because if it dies it will shrink to haf its size and then reliece a toxin in to the water. If it starts eating it is ok they are quite nice when they open there backs up fully.

Bill