I'm not really certain what the cause of this was, but I did want to add a thought ...
All of my starfish are dead. The red bali star's remains are nearly unidentifiable. He really did just melt. I'll post a picture after church.
I believe Reefrunner also hinted at what I was thinking ... mobile inverts, especially
stars, are *extremely* sensitive to rapid changes in salinity, temperature, and pH. This is why many ppl drip acclimate stars for several hours. Too-rapid acclimation would produce almost exactly the symptoms you have described.
I'm wondering if somehow, your pH dropped or spiked (possibly along with temp/salinity). Intense pH fluctuations could also cause problems with your fish.
Not sure if this is helpful, but if you are 'reacting' to the problems by doing large water changes, make sure that the temp/salinity/pH match exactly, or you'll be inviting more problems.