Not much you can do at this point. Leave it in the current tank as a move to a different system can only hurt at this point. (Salinity and/or pH shock might finish it off). Try to figure out what attacked it and see if you can't remove that fish to another system until the star has recovered to the point that you can safely move it. Starfish really don't get "sick," and under good conditions you can reasonably expect it to regenerate the lost tissue.