It can be brown algae ( check your light levels and your phosphate levels ), it can also be diatoms ( water too rich in silcate ), but diatoms also need the light and the phosphate. When you filled up the tank you filled it tap water ? you checked the water quality parameters ? you replenish the evaporated water with tap water ? you can also be overfeeding, if the tank is well cycled you won´t detect ammonia, nitrites and nitrates but phosphate levels increase slowly ( the phosphate comes from the food ) if you don´t perform regular water changes. You can reduce phospate levels with resins in your filter but it´s only a temporary measure.

I do weekly water exchanges with water from my aquarium shop, I do about 8-9 gallons a week. I am leaning more towards diatoms as I couldnt find a pic on the net of what I have and the brown algae pics I saw dont look like what I have.. My setup has been weird from the start, I once had a huge jellyfish outbreak, which went away eventually but the other day I noticed a few more returning.. I now also have about 10 aiptasia's but will be killing those when Joe arrives in the mail.. I do feed alot but water test all came back fine.. my salinity was high for a while but is now back to normal level.. I dont really have any corraline algea on the back walls and never have which the aquarium pros are a bit puzzled by.. I also had a huge coral die off as well a few months back which was never explainable... not sure what to do about the light as I cant really change it; well I could change the bulb its currently a 12k.. I also have the buffer stuff that comes in a bag down in the plumbing which is supposed to help filter the water.. I do have purple algae on the back walls but it looks more to be the long hairy type and not yet corraline.. and each water exchange I do add a couple drops of the Luigis solution.. not sure what else to do besides keep on scrubbing each water exchange.. attached is my site, all the pics are old, but a few close up pics of the fish you can see some small dots, this is when it just starts and then gets progressivly worse.. check the second yellow tang pic, I think that has the best shot of it..

Here is a link that might be useful: my tank

I would get rid of the trigger fish for starters, those are not invertebrate safe, I would check the phospate levels ( specially after the coral die off ) and I would add 3 or 4 turbo snails and a dwarf angel fish ( coral beauty, bicolor, potters or flame ), not all surgeon fish like to eat filamentous algae, yellow and blue tangs don´t like it ( dwarf angels do like it and when they run out of it they accept other foods with ease ), turbo snails keep the tank incrusting algae free and they don´t eat other types of algae in my experience. Your tank is too small to produce enough algae to maintain a yellow eye or chevron tang to keep the brown algae under control. Maintain your water density at 1.024-1.026 ( 32 grams of salt per liter of water ).

Its not algae is diatoms.. I would love to remove the trigger as it is a pain in the butt fish.. it has killed off anenomes and other corals... the die off was months ago and the tank is thriving now.. but still no corrilane algae on the back walls.. some on the rocks though.. anyway some day will probably have to remove him and maybe my snowflake eel too but will have to wait and see.. anyway I think I am good for now.. thanks for your advice..