They should get more LR. What kind of skimmer is on the tank? Personally I wouldn't be using a filter, unless it is filled with LRR. Sounds like a cyanobacteria outbreak. How long has the tank been up and running? All SW tanks go through that, and it will go away, but if it has been up for more than 6 months than I would recommend a couple mexican turbo snails. Cyano is similar to a FW slime algae, both thrive in a high nitrate, or high phosphate condition. If you are running a standard filter with bio-media that would be the cause of the high nitrates. If you have enough LR and a good protein skimmer, I'd just get rid of the filter, or dump the media and fill it with LRR.