Hey Craig
I have had a few of the bright red starfish that your talking about.
Both of which lost a limb when first introduced into the tank.
This is a sign of stress in most cases and is not a concern if it's just one limb, as stars can get stressed easily sometimes. Your salinity or PH was probably off slightly to the water which it was prevously in, none the less- not to worry. However if it continues to loose limbs then this could be a sign of some kind of water quality swing, disease, or starvation. I had both of these stars for about a year each, but I think with time in a small tank such as mine, then simply ran out of food.
Goodluck, Rich
