I've tried some super concentrated potassium permanganate on rocks to get rid of some nasty algaes (turf is one) and I found it didn't do much unless I left it in for a really long time (days), which unfortunately for corals would be a tad too much time
Of course it might be more effective against other algaes.
I've got no inside intel on that Tony. Haha. I've wondered if there was anything we could do for a long time. Never figured anything out. Excel may be an idea though. Never brave enough to try.
I do have a chemical engineer father who killed bugs and plants for a living who is married to a botanist who killed plants for a living as well, I'll inquire with them and see what they say.
The algacide with excel is weird. It's also a food source to higher level plants. Excel is just 2 things. Water and a polymerized glutaraldehyde compound.