Which particular shrubs Wendy?

Are you sure its mould and not lichen? As you haven't added your location to your profile we can't see whether your area is likely to have lichens or not. Mold is usually white and powdery looking, lichen is usually more fluffy looking and a dark green/brown/black sort of colour.
Another possibility is eggs of scale insects so if you can add a photo it would help identification if this info doesn't.
Lichens are just a sign of clean air and are harmless but the other two possibilities do need treatment.

Welcome to Grows On You Wendy! I hope we will get a photo of the problem to help sort it out for you :) Karen

Yea, a photo is actually essential in this case I'm afraid, could be a number of things.