Is it actually a scum? Can you brush it away and reveal smooth clean surface beneath? If the PH was really 5.0, that's pretty low and potentially could have damaged the surface depending on how long it was at that level. Since you said, "Ph meter" and most tests can't read PH even close to that low, can I ask how you got the measurement? It's possible that the reading of 5.0 was way wrong, and so it's hard to say. It's extremely difficult for us to help you without more information about your spa and the situation and without accurate test results. This forum is like a kitchen and the experts are master chefs; but if you provide bad ingredients (inaccurate test results), you'll get bad food (poor advice).