My pyrocanthus is covered in a grey-green mould. Help!

If, as you suggest, it has been in a container for "years", then its probably outlived its span in a container - you could try cutting it right back, turning it out of the container, renewing the compost and replanting, though probably its roots are filling the container side to side by now.
You could also try spraying with a fungicide, but it will only recover if its happy at its roots, and it doesn't sound like it will be.

Is it mould or lichen? Lichen indicates a poorly growing plant. It needs to be released into the garden (or someone else's garden!).