It does look like diatoms to me as well, which are common and very much benign apart from the ugliness factor. There may also be some film (micro)algae on the side glass, which is also not at all unusual (or unhealthy).
Not to put a damper on things, but as others have hinted, the ugliness may only be beginning. It takes many months for most aquariums to begin to reach equilibrium, and in the interim there will be many stages of change, and many of them can be pretty unsightly. Good teaching moments, because they are all part of a natural process, but these changes can be very distressing for impatient humans.
Is there a protein skimmer in the system? Even with no fish or other larger animals, proteins and waste are being produced all the time, which if not removed will ultimately feed undesirable organisms or allow overgrowth.
I agree that you probably need to ensure some good/better water circulation. It's much more important than many people realize.
Vacuuming the sand is a waste of time, only treating a symptom while not addressing the real issue(s).