This just started growing in my tank

[DWittman]

Does anyone know what this is ? Grew slowing out of nothing.

 

[Dr. Reef]

I believe it's nemostoma.
A red marine algae. Harmless but can take over. Some fish like tangs will consume it in no time.
Not sure on the kind/name but red marine algae for sure.

 

[cristata.reef]

Nemastoma is correct