When you mist your plants as often as I do, and you don’t use rain, or distilled water your plants will end up with water spots.  I get rid of them with a product called Scrubbing Bubbles.  I spray it on and wait for about a minute and then shower it off.  The result is that the water spots are gone and you have a plant fit for a show.  Only buy the product that is not Anti-bacteria.  I have attached a photo to this post of the bottle that I use.  I have also attached a photo from this weekend of Hoya megalaster, which I have sprayed with Scrubbing Bubbles many times before to no ill effects.