They will eat just about anything, but I suggest things that are in more solid hunks, rather than stuff like frozen brine. Shrimp pellets are my personal fav, but silversides, krill, shrimp, squid, algae wafers...they will typically all be eaten. As for amount and frequency, that will depend on a number of things, like the size of the brittle, water quality, etc.
I agree, BTW...avoid the green brittle, a known predator. It may or may not be a problem, even if fed. But, frankly, all brittlestars and serpentstars run this risk, and will certainly finish off anything weak, stressed, dead, dying.
Remember to acclimate them very slowly, and to have specific gravity around 1.025 with everything else within ideal range.