brown 'algae' is usually diatoms and not algae. This is a new tank right? (less than 2 months old?)
If so, diatoms are normal, and really aren't controlled by light exposure. They have to run their course. Some snails (and maybe even hungry hermit crabs) will eat it.
However, if it's more like a thin sheet, possible with little air bubbles trapped in it...that's cyanobacteria, which can range from red, maroon, brown to almost black. Cyano
forms int he presence of low current and/or increased phosphate levels (phosphates are why many people stick to RO
water for marine tanks...its phosphate free)