I have just bought some compost but it is abit mouldy, can I still use it?

Harmless fungus!. but do not use it on small seedlings.

Mine often has a fine white fungal growth abit like you get in mushroom compost. but as Dr bob says safe to use .

I personally would take it back to the shop where you bought it and ask for a refund. Some compost bags can be laid lying around for too long at some places and in my opinion shouldn't be used. Just my penny's worth!