Sorry to break it to you, but I think the starfish is on its way out.
My blue linkia had that happen, and it died within weeks.
Does the starfish has a slightly shriveled appearance? Mine did that along with the leg loss like that.
From what I've read (unfortunately after the fact), Linkia and other ornamental starfish do not typically do well in aquariums unless they're really big (in the 300+ gallon range).
Tyler