Its fine bust it up with a hammer put it in a bucket and just slake it down with water, the mold won't hurt it. Leave it in the bucket for a couple of day til it is mush. Get an old pillow case put the mushy clay in it tie it closed with a string. Either hang the case up to air dry or place the closed caseful of clay on dry concrete out of the direct sun and allow it to dry to a wedgable consistency(general take 2 or 3 days check it a few time a day to know it drying rate). Wedge it up and bag in a thick garbage bag.

As pea said, it is fine to use....it won't hurt the clay at all...BUT, I find, depending on how you plan on glazing it, that you want to make sure you put it through a slow bisque firing first to make sure that all of the organic impurities come out of the clay...they can cause bubbles in your glaze if they are not completely burned out...and take the glaze firing slow too...so that any higher temp residual material can escape while the glaze is fluid. Safety first as well..wear a mask when you break it up..silica dust from the clay is bad enough..who knows what kind of mold that is!