I would be wary of keeping it in that tank - by the time you may notice a problem, it can be too late. It is better off in the 125.
As a general rule, they can not be spot fed, and require lots of mature LR. I would try but most do not eat it. What they are eating on that rock is open to debate, but it is quite clear that their survival rate is pretty closely correlated to amount of LR available. They do not appear to eat "algae" per se.
I haven't seen a real blue Linckia that small...do you have a pick?