What you are seeing is the talc powder used in the manufacture of the bands coming to the colored surface of the latex.
It is normal in time and use for bands, especially if they haven't been used on a regular basis. The whiteish color you are seeing doesn't affect the integrity of the band and is just a cosmetic imperfection.
You can clean some of this white coating off and recondition the bands with a product called "Black Magic Tire Wet Spray". This can be found at most automotive supply stores and online. You simply need to apply to a rag and wipe down the bands once a month. Here is a link to an article that shows you how to use the product with the bands:
If your resistance bands haven't been used in a very long time it will be a good idea to inspect the bands very closely for any kind of cracking or splitting in the latex. If you see any cracks or splits you should not use the bands and contact us for a warranty replacement.