It is really VERY important that you provide SPECIFIC tank parameters.
It is unlikely, IMO, that this star is being attacked and is otherwise healthy.
The slow disintegration of the arms, or just arms "falling off" is characteristic of several issues.
If the star is less than about a month old, it is characteristic of acclimation shock.
If you have had major water quality fluctuations recently, it is characteristic of osmotic shock.
If the star has been in the tank 6 months to perhaps a year, it is starvation. This star does not typically take to spot feeding, and does not eat "algae or detritus" or scavenge. It needs live rock, and huge amounts of it.
In any of these cases, shrimp and hermits, etc will be attracted to the star as they smell it is dying.
This star, though small, needs a very large mature reef system to stand a chance of surviving. On the order of a 100g with at least 125lbs of LR minimum. It is at least as, if not more difficult to keep than many Linckia. The small size does not make it suitable for smaller systems.
The success rate with Fromia stars is very very poor in captivity. Please provide tank parameters so that we can try and provide the best care. DO NOT move it to another system. Isolating within the same system is good, but I suspect it is too late.
Please also provide tank inhabitants. But I would say it is FAR more likely and common that this star is dying. That is, unfortunately, the norm for most.