if the star has not eaten anything for a while (not necessarily in your tank), it will obviously starve. unfortunately often when they starve they dissolve from the inside out so you dont notice until it's too late. by the time the exoskeleton is rotting from starvation, there isnt a lot of hope left.
if it's not starvation and just injury or disease or something, then yeah i would try the cutting method like christy suggested. keep an eye on water params
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