Sounds like Diatoms. The link below might help.
https://www.reefcleaners.org/nuisance-algae-id-guide
Here is a picture
I removed the chaeto because it was probably releasing the phosphates and nitrates back into the water. I had a strong light on it, maybe it was too much. I will try to put more in once the tank starts to mature a little more and gets going. Other then phosphates, nitrates, and light, what else does chaeto need, this was probably the problem.yep.. totally normal diatoms..
Chaeto is potentially lacking sufficient nutrients.. maybe light.. (you didn't give us much info to go by)..
If the Chaeto is dying...remove it. You don't want it polluting your water.
I removed the chaeto because it was probably releasing the phosphates and nitrates back into the water. I had a strong light on it, maybe it was too much. I will try to put more in once the tank starts to mature a little more and gets going. Other then phosphates, nitrates, and light, what else does chaeto need, this was probably the problem.
Just to add:
SOMETHING has to grow on those white rocks. Brown diatoms first, then maybe cyano, then maybe green hair, and eventually green and purple coralline algae and sponges, etc.
Iron will help it but you have to be careful since it will also help other algae.
The chaeto died because there isn't enough available nutrients for it to grow. The tank is to new basically. Let it run for a bit, once you add a couple fish, wait a bit longer, then retry the chaeto.