ID help

[Dr RBG]

Found these brown flag like things on a rock are they good bad or ugly?

 

[saltyfilmfolks]

Looks like bryopisis. It's not good.
Try to get it out of there. If you can , do it out of the tank.

 

[Dr RBG]

What causes this. Should I take the whole rock out

 

[saltyfilmfolks]

All algaes have to be introduced.

If you can easily take the rick out and remove the effected area by scraping or using a knife , it may not spread.

If it gets into the system more you can use a product called fluconazole.
But currently it's pretty minor.