Well, it may grow back, or it may not....
It takes a lot of energy.
Also note that most hermits don't attack without reason- often the star is dying. They lose arms due to stress. How long have you had it? How did you acclimate it? What are your water parameters? What is your specific gravity.
I would assume in fact the star is in trouble, and the hermits are taking advantage of this.
Sandsifter stars in general do not do well in our tanks. Even if it was healthy, its long term prognosis, IMO, is not a good one.