Because you have new rock and sand, until you get a nice biofilm on them and build up your biome you will have diatoms. There is nothing wrong with diatoms many of the cuc you add will eat them. Make sure your lights are on and they will start growing with is a sign the tank is maturing. Add cuc and blow them off the rocks and you can keep them in check. Once you see the rocks starting to grow green film algae you will know you are getting over the diatom stage. I would also recommend seeing if you can get some sand, rock, cheato or bio media from an established tank. This will add copepods and amphipods, bacteria and other beneficial items to the tank and help you with bio diversity. Add it to the sump if you are concerned about algae or aptasia. It is inevitable you will add these over time unless you have a quarantine tank for you cuc and corals so I would t get too concerned.