Try feeding a variety of fresh seafoods...I'm not surprised the seaweed didn't work out although since they've rarely been kept successfully it is good to try anything and see how it goes. While their natural diet is in fact biofilm, it's untrue that no Fromia genus stars will eat any alternative foods. This is something I know from experience. However, it does depend on the species what foods they will take.
It's great that the spots have reduced! At this point I would focus on finding a food source he will eat and worry about the leg out of the water and possible reasons later.
