A Pin Cushion urchin will help. I would be a little hesitant to add more than one unless you plan on removing it once the algae had been cleared up to avoid possible starvation. How large is your tank? Urchins will also consume coraline algae so that may be a consideration for you.
I see you have a snail and hermit crab in the picture. More of these can be added as well.
How old is your system and where are you keeping your nutrient parameters?