Fast growing algae?

So as I have said before, we are brand new to this. Days new.
On Saturday we had our tank delivered, set up, added the salt and the RODI water.
On Sunday we kept retesting the salinity and got it to a point of 1.023 so we added the live rock and live sand.
We have not tested for anything else because we were going to let it 'cycle'
Yesterday, Monday, we looked in the tank. There is green algae hair growing off of the live rock, and its moved onto the glass.
The tank and set up is only 3 days old. How is it possible that this is already growing too fast. I don't think it 'came with the LR' because when we bought the rock we actually made a mistake and bought it too early on Thur and had it sitting in its bag with some RODI water, so from my understanding a lot of it died off while sitting in the bag for an extra 3 days.
The Algae is green, grass like, and is growing fast. Too fast????
We have NOT had the lights on, it is NOT near direct sunlight.
We just have the pumps going, the water sitting, and just left the tank alone, like I said we have not tested for anything other then salinity yet.