The first picture appears to be stippling from spider mites; however, if thever first and second are the same individual plant, it appears to be mealy bugs. the third picture is cold damage. This type if arrowhead plant is not hardy here.

What do you recommend to get rid of mealy bugs on indoor houseplants?

Here is an Extension publication on Houseplant insect control. It covers mealybugs as well as other common insect pests: http://www.extension.umn.edu/garden/insects/find/houseplant-insect-control/

If you aren't successful eradicating the mealybugs or don't want to bother, you may want to throw out these plants rather than possibly contaminate your other houseplants. Any time you receive a new plant whether from a garden center or a friend who shares, it's a good idea to quarantine the new plant to be sure it is not bringing in any new pests (insects or diseases) that may infest your other healthy houseplants.