Your tank should have passed the brown diatom bloom stage, most new tanks get it, but with the flounder, it just may have taken longer to finally catch up. Google brown diatoms to view images to be certain that's what it is, but I'm fairly certain that is what's going on. Diatoms will eventually go away on their own 95% of the time, especially if you maintain good water quality and keep up with water changes. They eat all the silicates in the sand/substrate and then die off once their is no more silicats for them to eat. Keep in mind that if silicates are present in your water, such as when using tap water, they will constantly have something to eat and won't die off. If they do not die off in the next two weeks, order a Salifert Silicate test kit to test for it in your tank/water. They naturally occur in all new tanks, but do look gross while they are there. Some people GENTLY stir the sand which I don't recommend, some vaccuum them out (I do) and most just let them run their course.