I find Diadema urchins surprisingly easy to spot feed if needed. Take a turkey baster and shoot some Spirulina flakes or pellets (Spirulina is algae) towards it, you'll be surprised how adept they are at catching stuff from the water column if they are interested in what you're offering.
That said I hardly ever target feed mine and I have an algae free tank and yet it doesn't seem to starve. I do feed Nori everyday for the fish and I know he (the urchin) will occasionally belly up to the nori stick and eat his fill. Doesn't happen everyday though and when it does happen it's always at night.
HTH
