Wipe/scrape some off and put it in vinegar. If it bubbles, it is lime scale. If not, it’s just salts from the evaporated water (e.g., sodium, potassium, chloride, sulfate). I consider water with GH 7 to be moderately hard, not soft, so I’m betting on lime scale.
Yeah, you're right. It is moderately hard. I was mistaken. Maybe I should be shooting for a lower gH.