Its been awhile since I had my saltwater tank, but maybe I can help.

Do you have the tank's lights on? When I cycled mine, I always had the lights off because I didn't want algae to grow. The lights don't need to be on while your tank is cycling, and algae shouldn't grow if they are off.

Like I said, its been awhile since I had my saltwater tanks so I could be wrong! Just though I'd try to help.

Are you sure it is brown algae? If it is more like a rusted brownish color, your tank was exposed to way too much light.

Is it an established tank, or are you still cycling it?

If it established do a water change and get rid of about half the water, and get the magnet cleaner for the glass. If you are cycling invest in a protein skimmer and TURN OFF THE LIGHTS!!!

How about diatoms that attach to my acrylic tank (brown spots and streaks).. I have to do a hard cleaning each week when I do my water changes to clean it off the front of my tank... weird as it seems to not affect the back and side walls and only the front.. the scrubber magnets arent strong enough to remove the growth.. it takes me about an hour each week to do the water exchange and cleaning of the front.. not to mention my clown bites me constantly while I am scrubbing.. my wife thinks its funny but that little suckers bite hurts.. he actually jumped out of the tank yesterday trying to attack me..

Please go visit saltwaterfish.com and you will get a lot of the info you are looking for. The brown in a new tank is diatoms and is a normal part of the cycling process. If you are still haveing issues with them later then, you are most likely using tap water for topoffs and water changes. Use only RO/DI water and watch your water chemistry. Amm., nitrites, nitrates,phosphates, and others.
kevip, is it brown or a purpulish color? Their are a lot of different shades of coraline algea and if it is a crusty hard instead of soft I'd say it is coraline.
Pattie

The algae is smooth and slick to the touch.. its not hairy and usually fills in the scracthes in the acrylic.. then spots.. been a pain to clean ever since I got the aquarium and has caused me to scratch up the acrylic cleaning it.. tank is well cycled even though I dont have any coraline algae on the back walls.. only a small bit on the rocks.. tank is over a year and half old..