Grind and smash the Coralline algae
 

Not sure if it works but I get some coralline algae from a variety of other fish tanks and stores. they scrape the glass and I take the coralline algae smash it and grind it into a million pieces and spread it all over the tank. I heard that different types from different places mixed together is a good thing. Been doing it for years. Also I too have 2 large Urchins and they can't keep up.

Magnesium is dosed and use lots of purple up.

Hopefully this helps

thanks
erick