Insane algae outbreak.. new tank

[JDigital]

So I picked up some live rock about 5 days ago.

Things had been looking good, but last night I noticed a small amount of long stringy algae, not quite green, but not red... sorta brownish/green... today I get home and find this!







It has come out of NO where... The rock is about 50/50 Fiji and Tonga..

Ammonia is about 0 (0.1-0.3) Nitrite is 0, Salinity 1.024, Instant Ocean Salt, Tap water (good quality, no signs of this algae in my roommates tank who is using the same water), 1800+gph flow rate. No dosing of anything. not sure on pH or Ca right now..

EDIT: It actually looks worse in person than it does in the photos.

This is the rock a day after adding it.... Super clean, no problems.