If it is diatoms, it is a bacteria and not an algae. And the thing with them is they will still be there, just dead. Really they are one you should probably just let run its course. But any fish safe algaecide would do the trick for an algae. Hydrogen peroxide works for bacteria, but you would have to turn off your filter because it will also crash your cycle and they would just come back after treatment.
So I should just leave it there? Will it go away? l did a total clean out and wipe down of the tank, rocks, decorations and outside of the heater, thermometer and filter unit. It just came back. It looks awful.