Mould, definitely. The other version is a colony of yeasts/Fungi. Of course, if I had been brought up in America my answer might well be the other way round. It is one of those things divided by our common language.
I always used mould till a few years back, then started spending time on american boards and got confused. I'm currently making an effort to re affirm my Britishness. The term Strike Planes is also being used in the cause.