Cactus growing mould? Help!?

My indoor cactus has recently grown white fluffy mould. One of my others had already got some on from being watered from above accidently. I know for certain I haven't wad thisone from above. All the cactuses are small and watered every 1.5 to 2 weeks. The one I am talking about is green with thick, hooked sort of spines. It has the white furry mould covering the top of it. Can I wipe it off and how can I prevent it?