I'm inclined to say diatoms, though I'm no authority at algae ID as I'm still a n00b. I'm currently seeing the exact same thing in my newly planted 125g. From what I've been reading, it is common in new setups, low light, and/or excess silicate conditions.
Ottos, and BN plecos will eat the stuff.
I'm going to give mine a few more weeks before I start to enter "panic" mode myself :hihi:
Whatever you want to call it, and frankly there are so many hundreds of algae species that it's always tough to be sure which you've got, I've successfully treated algae like that with manual removal and an increase in CO2 and/or Excel. Seachem Flourish Excel does wonders!