Brown "algae" is pretty normal for new tanks. This is commonly diatoms. As slica levels are high in new setups this variety of "algae" proliferates in new tanks because silica is a food source for diatoms. Phosphates are also a food soruce. While silica levels will diminish as your tank ages [because the diatoms will eat up the excess silica] phosphates are commonly and continuously added to the tank by us, the hobbyist.
Sea salts frequently contain phosphates, including IO, even though the label on the salt bags say no phosphates, most have phosphates. Additionally water used in the hobby, epecially tap water, a lot of times has phosphates. I'd suggest that you test your sea salt mix and your water source for phosphates and try to reduce, if not eliminate, any phosphate sources you yourself are putting into your tank.
Also, the cleanup crew you have is insufficient. You need a lot, lots more.