Yep, probably a little diatom outbreak. Can't explain it but I did see this more than once in the first 6 - 8 months of my own tank.
Eeek! Had some other brown crust for a while too. Diatoms, well, they scrub off easily. This stuff... did not scrub off no matter what. If you have something like that, we can talk more about it, but the most important parts of this story are... despite agonizing over looks, not being able to clean it or identify it, not being able to stop its spread... eventually it did stop spreading and then one day, like magic, it started to recede. What grew wherever light directly touched, and stuck for more than six months, disappeared over a matter of weeks and never returned.