Slightly measurable PO4.
Some macro's in the refuge will help.

Absent PO4 will cause some corals to grow slower, the algae inside them needs some PO4 sources, generally from the food sources the coral animals catch, so in general, happy animals, happy micro algae in the coral= happy colors/overall coral health.

I'd not suggest dosing PO4 to anything other than a well run macro algae tank etc and then .2ppm or so, or less even.

It's nothing like the FW planted systems. Many algae have adapted to limiting PO4 levels, unlike FW systems.

Regards,
Tom Barr