Yes, cowries eat live rock...or, rather, the coraline algae on LR. Don't panic and start ripping your tank apart to fish out the poor cowries. This is a good thing, actually. They eat very slowly...you can imagine how hard it is for a snail to eat calcareous material. They will clear small patches of the coraline and, when finished, they move on to eat corline elsewhere. In the meantime, the area that they cleared is left for new varieties of coraline to grow, thus stimulating variety/diversity in your reef ecosystem.