I would probably get an iodide supplement to get that up. I mean, it might not be the answer, but it's a start. After all corals house algae..

Over the years more and more I see people talk about a drop of iodine every week/two and positive results.
I'll post the links later, but I have found multiple statements talking about 'corals DO use iodine/iodide'. Of course it's not been researched to the extent many would like to call it 'concrete', but iodine is a pretty important compound and a requirement for many, many organisms in the world.
Hopefully things turn around for ya