It may just be the organic matter in the compost 'rotting'. Sometime bark mulch forms a white mould, but to my knmowledge its had no bad effects. Any chance of a pic?

ive had this problem with some of the bags i bought, i think it depends how long the garden centre has had it on sale for.
i bought some new bags the other week and they were a lot better than the ones i bought a month before that