I feel your pain... I've got..er, had..a pretty good diatom bloom going after 6 weeks of cycle. I did a search here for "brown algea" and "diatom" and then read for a couple hours.
From all the reading, what I took away from it is this: Its part of "life" and it should go away in a few weeks if you are using RO/DI water.
I changed my filter element, which has activated carbon. I took it a step further and threw in a bunch of little snails. They are doing a super job of eating all the diatoms off the rocks. I also added a sand-sifting star, but he's pretty...well, lazy... so far. But he just went in yesterday, so I'll give him a few days to adjust. If I'm still unhappy in a few weeks I'll look at a phosphate
filter element, or bag to help get phosphate outa there.
Hope this helps.