Both of the "cultured marble" vanity tops in our bathrooms have a layer of white powder on the underneath surfaces. We just noticed it when we dropped something on the countertop and this powder appeared in the top drawers. Reaching under and rubbing with fingertips causes the power to get all over our hands. What is this stuff and is it a cause for concern? Should we try to clean it off? I am not sure how old these vanities are but I'm guessing 5 - 8 years.