I have two of them, but I've never seen anything like that. But, mine hide in the rocks with only an arm or two showing so it would be hard to see anything like you describe. I was going to guess that the sea star was reproducing. But after reading a little, I'm not so sure.
From Wikipedia: "The gonads are located in each arm, and release gametes through gonoducts located on the central body between the arms."
It might have been a pimple due to an infection, or it could have been something that is normal and inocuous. There is probably little you can do regardless of what the pimple was. _________ -- SAW39 45 gal modified hex. Started and cycled August 2005.
2 Serpent Stars, Ocelaris Clown, Yellow Tang, 2 damsels, 1 huge bristleworm, Macrodactyla dorensis anemone, Dardanus pedunculatus hermit with an anemone on his shell!
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Starfish pimple? 
