it's diatoms. Normal part of cycling a tank.
Keep walking past the tank, keep you hands out of it, and practice patience. You will see over the next week-ish the bloom expand then contract. Once contracted do a partial water change and you should be ready to add inhabitants.
The most success I've had with a new tank was with all dead sand and rock. You minimize hitchikers and unwanted parasites, offset is it takes longer to establish and mature. Adding a seed rock or sand from another tank will shorten the wait, but you may end up with some undesirables from the host tank.
The best tip I ever heard was 'Nothing that happens fast is good'. Bear this in mind and while you are waiting on your tank to mature, set up a quarantine tank to keep your new purchases in. Quarantine tanks should cycle, but you can add the bacteria supplement and perform regular water changes to offset and prevent any spikes in ammonia.