A few years ago I needed to make masters for stone pavers, and wanted them to look really old and worn. I made a plaster mould, oiled it lightly, then made a thick mix of ordinary flour (it may have been self-raising, can't remember!) and water, painted it very thickly into the mould on top of the oil. Allowed it to dry before pouring in plaster. The flour reacted with the plaster, giving a very pitted surface to the final cast, which looked convincingly old.

The main point I wanted to make is that the plaster casts went VERY mouldy VERY fast - which could be useful for starting off the greening process, getting a mossy look fast.