Identification does not matter in invasion threads, it causes hesitation in fact
what matters is acceptance vs rejection of the invasion. you have a way to make that sandbed completely free of that algae, regardless of it's species. it doesn't look unadapted to the tank either, it looks like it can choose when to rule your real estate pretty much any time. running the light windex blue even though it doesn't look ideal, lowering overall intensity vs how it runs now are first two steps, don't reinstate this level of lighting until its packed in corals and the invasion is gone.
that's not how I would fix your upcoming 5 month uglies phase, but given the amount of work that would take most won't opt for a rip clean of the sandbed, they'll nearly always choose the extended invasion holdup. it's the best low work change you could make to help your tank: lowering light levels and bluing them up for months, never that color above. that is a helpful selector of your beginning invasion.
dry rock tanks need manual controls, not chemical ones, to turn into coralline reefs.
if that was my reef, it would be ripped clean by 5 pm today and look completely brand new top to bottom, no hesitation /my investment is worth it/ then from that clean condition I'd weigh preventative options, and only from that clean condition.