Ugh, Staghorn algae, my nemesis.
This type of algae shows up in my tank when the plants stop growing well, and the only way I've been able to kill it is to simply remove as much of it as possible manually at the same time as diagnosing what is the issue with my plants. Once I've corrected my plants and they start growing well again it turns white and disappears until another day.
Things I do when I start seeing this algae:
I look at my maintenance, have I been up on that for example: weekly 50% water changes and shaking out the sponges every time. If I don't do this it allows the algae to take hold.
Are my ferts still good, IE did my auto doser run out of anything.
Do my plants have any deficiencies that I can see?
I've tried everything in the books to kill this algae through chemicals like glut/excel and H2O2 and nothing has worked for me besides making sure my tank is healthy and manual removal.