If your tank is a new setup-not to worry, as brown diatoms are part of your cycle. I would stay away from
any antibiotics in a reef tank :!: Instead use bottom cleaners such as hermits, a few types of snails, cowries, and conches will clean up the stuff naturally
A bottom dwelling fish such as a goby for example are also good, as they stir up and clean your live sandbed.
Goodluck and Happy Reefing
cheers, Rich