Feeding an Urchin?

[Oscar]

When I stated my tank just over a year ago I inherited an urchin with my live rock. A few months later I noticed a baby urchin the size of a dime of the same species.

Both of these guys a pretty good algae grazers. But now that my algae is under control they ae running out of food. They are getting more aggressive by moving rocks and knocking corals off of their perches.

I tried feeding nori attached to a rock. The hermits and snails loved it but it turned to mush very quickly. Are the commercial algae sheets more substantial than nori?