As you can probably imagine, plants lacking chlorophyll are typically at a disadvantage, since chlorophyll converts light into energy for the plant to be able to grow. However, "albino" plants (that have little to no chlorophyll) are not unheard of - this article from National Geographic talks about albino redwoods that are able to survive by feeding off of their healthy neighbors!: white wonders

Chlorophyll can also be missing from only a few parts of a single plant with healthy leaves (which sounds like the case for your hoary cress). This can be caused by nutritional deficiency or a genetic mutation.