Inside or outside?
Vinegar dissolves hard water deposits on a dry surface, but be careful not to get much of it in the tank. I wet the deposits with full strength vinegar by slapping a vinegar dipped paper towel on the deposit, let it soak for a few minutes and then if necessary, scrape them off completely with a single edge razor blade.
Inside the tank, the deposits should eventually dissolve away and rub off with the action of scraping algae.
Unfortunately, some things cannot be removed. My current reef was a friends 20 gal tank that was going to the dump, if I didn't want it. It had been used for freshwater and was not kept full so it has terrible etching from the hard water deposits. That etching will never go away so the worst side is the back and I just look past the etching in the front.