He may just have experienced shock. Many fish stores have odd water parameters, so you need to adjust saltwater creatures a bit more carefully than freshwater. As well as the temperature acclimation done via the float method, you also need to do a chemical acclimation, by slowly adding some of your water to the bag, until the bag is at least 50% your tank water. The best way to do this is by starting a siphon in an airline hose, and then tying a knot in it so that it just barely drips into the container with the aclimating creatures. When the amount of water in the container has doubled, you can net out the occupants and dispose of the fish store water.
How is he doing? Is he moving around ok? The wounds look like they're healing?