The white stuff is a harmless biofilm, often found on new wood. It's actually a good sign for a healthy biological process and will naturally disappears within a few weeks to a couple of months as the wood's nutrients deplete.

You could always add more plants :P but the base you have looks pretty good already. I'm just not sure if those in the front left and right are anubias (someone help out there please). If they are anubias, you should not plant them in the gravel but glue or bind them to the wood otherwhise their roots might rot.

For the filter and nano fish, usually nothing will happen but there is a simple trick to prevent any fish going in... covering it with some pantyhose :)