As Lapin said you need to get a better supply of water. If you have local fish stores near you they usually will sell rodi water to you. Worse case the big box pet stores sell jugs of it but it is more expensive. But in the short term it may help. If you are in an area that is in lock down, you can also look for a walmart that sells ro water by the jug or worse case as Lapin suggest distilled. Not the best but better than tap.
Once you have water ready I would try and siphon them out. Replace the water that is lost with fresh salt water and then go 3 days of darkness (wrap the tank in something if there is a lot of ambient light. especially if it gets sunlight).
Diatoms are photosynthetic and reproduce by cell division. So the more you are able to remove the less they are able to reproduce. You may need to do this cycle of siphoning and going dark a few times of 3 days on and 3 days off. If you have corals . If you just have fish you can turn off all the tanks like and you can go with ambient light for a while, but block out an sunlight.
Also get some CUC as Joe01813 suggests. Try to find pre- quarantined QT from a reputable source. (I QT everything wet going into my tank)
The good news is usually goes away as the tank matures. Its common and it is a issue that gets refereed to as part of the ugly period.
Do a search on diatoms on here and you find some more info.
How big is the tank, whats in it now and what are your parms ?