+1 to Kien's suggestion for biopellets, or some other form of carbon dosing. I think they're making biopellet reactors small enough for a 28 gallon tank these days.
Fixing the problem will likely take a while though, as you need to both exhaust your bank of nitrogenous waste that your rocks are emitting (if that's what's happening), and get the nitrate reducing system up to a capacity that can handle the amount that's being produced. While that happens, you might end up with a cyano explosion (common when people start any form of carbon dosing), and the other algae will keep growing, so it might seem like it's getting worse before it gets better. The key will be to keep removing as much of the algae as possible, as routinely as possible until finally the nitrate reduction system starts to outcompete it and it stops growing back. Depending on your system, that could take a couple of months. If you just run the pellets or carbon dosing and don't actively remove the algae, you will wait a lot longer to see an improvement.
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