I've had several situations where my houseplants get a thin white dry-looking mold growing on the surface of the potting soil. I live in the desert Southwest of the U.S. Sonoran desert.
I'm diligent in my watering but show restraint and patience. The dryness can make you water more than you should but the watering frequency is certainly more frequent here.
I've had people tell me it's calcium build up but our water isn't hard.
If I can find a picture later I will update the question with an image.