So many options.

Water changes, how often?
What are your water parameters? If you don't have a test kit take a sample to lfs tomorrow.
High phosphate is a likely cause.
how often and what do you feed? Could you be over feeding?
How old is the tank? New tanks cycle a brown algae diatom all over the place that will cure itself.
Are your lights too old? bad spectrum and under powered lighting can slow down nutrient absorbing corals and plants allowing simpler life forms of algae to thrive while your tank "hibernates".

Just a few places to start. Natural predators can help such as tangs or blennies, loads of snails and proper clean up crews.