I assume this is a new set-up, meaning within the past couple of months? Diatoms is a common issue in new tanks, due to the fluctuating conditions. Once the biology settles, this should stop.
I should check to make sure this really is diatoms and not a form of brush algae which many take for the same. If it easily comes off with your finger, it is diatoms.
On the pH, let it alone. You have soft water fish so this is not going to cause the Betta any harm, quite the opposite. The pH is connected to the GH (general hardness) and KH (carbonate hardness or Alkalinity). These tend to buffer the pH, depending upon their respective levels. The acidification of any aquarium is normal, and governed largely by the GH/KH along with water changes and substrate cleaning. Live plants and fish load obviously impact it too, as would the presence of calcareous substances. As I said, with soft water fish this is not likely to cause issues.
BTW, when testing tap water for pH, you need to out-gas the CO2 which if present will affect the result of the test as CO2 creates carbonic acid which lowers pH. This is the same process as what occurs in the aquarium itself, though for different reasons. You can let some tap water sit 24 hours before testing, or more quickly shake some very briskly in a covered jar for several minutes and then test. The GH and KH you can ascertain from your water supply folks, probably on their website; worth knowing these numbers so you will better understand what is happening.
Byron.