They eat pretty much anything so it probably depends on the amount of algae and organic material you have based on your bio-load.
I had one for 2+ years prior to corals and I usually fed mine 2-3x per week just 'cause it was fun. Krill/clam/shrimp/whatever but it would also go after nori and algae wafers that I put in for other inverts.
*had to donate mine after I started getting into soft corals -- keeping it fed worked for a while until it discovered where some of my zoa patches were, then it was game over