Here's one chicks opinion on algae: You can either fight it incessantly, underfeed your fish, run a phosphate adsorber continuously, scrub and fuss with it
OR
Give it what in needs in your sump, encourage it to grow there instead of the main display! Does your sump have light? If you run the lights opposite your tank (called RDR, Reverse daylight refugium), it will help maintain pH (that's a bonus!).
Can you get a large piece of the rock and place it in your sump?
You can put some base rock in with it, let it grow onto it, then put the rock back once the issue begins to resolve.
While you are waiting for the algae to start to grow in your sump, you can use a toothbrush to make it really uncomfortable in the main display. I also think you could add a few more herbivores to your main display. Hermit crabs love nuisance algae. A hungry tang will, too but not if there is something else yummier to eat. I love my lawnmower blenny to keep my display algae free.
I would also look into other algae species to place in your RDR. I've found bryopsis to not compete well with other types. My favorite is a red halymenia that my tangs think is candy. As I harvest it from the sump, they get a very nutritious treat. Once their tank is clean, they will suffer nutritionally if they are not allowed to graze continuously. You can also add a piece of nori, but I like to give my pets live!