this is my take:
do nothing this is natural. adjust no param reactively except for less lighting if you want, theres low demand for lighting as of now
I call it green dot microalgae, could be coralline agreed to but nonetheless this is uncolonized white base rock... in the reef there is a depositional order of operations when you input open vital space into the tank and then seed it with colonizers (not seeded yet here)
the early colonizers for your substrate are bacteria first, then algae and cyano and diatoms in any order happenstance might present. if this is green coralline and you've passed the first stages (it usually deposits after those, not first, coralline is usually a 2 mo + deposition) then pinks and red coralline w start to layer in time. since there are no spots on the substrate or walls I don't think its coralline I think its microalgae but either way the cause is the substrate not being already covered with coralline purple or coral flesh. its open for taking
