I would bring the Mag up to 1500 and keep it there for a couple months. Manually harvest the algae as well. If you don't notice a decline in the algae then you might want to try new bulbs with a colour temp of 15k to 20k. I like Radiums the best.
Do you use RO water for the tank make up water? If so, how old are the filters?
Urchins are very effective herbivores but they can be limited to where they can get to due to there shell. A Sea Hare might be an effective option as well.
Cheers,
Tim
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