You should get your tap water tested if you can (or go to your countys website you will often find the analysis there, eg. I know mine is 392ppm from their website, super hard fuck where I live). Hardware stores sell cheap water test kits too.
I agree though looks like calcium. What is the pH of runoff vs. nutes you feed, what is the EC of runoff vs. feed?
Can't hurt to give it some cal-mag, just keep an eye on EC.