you seam to be doing everything right. Cudos!

You probably have a fungal infection of the tank. If your fish and plants seem to be fines there is pobably no problem with it. If you want to get rid of it buy some malafix and take out the carbon in your filter. That should get rid of it.

PS your nitrates are slightly high, and a reduction of them through water changes and anti-nitrate add on packs for filters should help. With lett nitrates the white stuff may reduce.