I am sure a big part is the silicates... normal progression & good, eating & using them up
I recently added, about one pound of new sand to my tank that has been algae free for months & it turned a light brown for a few days... this was the 1st sand I had added since setup & I am sure it was correlated to the silicates. I knew it would soon go away
Cutting back on your light will knock it back as Terry suggested, but in the early stages for tank progression, not a bad idea to let it run it course, it will go away in time & you will have a pretty tank - not to worry (I did when I had it) but I listened to others here & the progression happened just as they said it would