its fungus. due to the warm damp conditions. Try scratching the surface to break up the surface. then let it dry just a little so that the fungus cant take hold but still wet enough for the seeds to germinate.

Why have you got your propogator in the airing cupboard are you growing mushrooms? if not you need light to grow most things, hence mould on the compost.

Agree with DrB your propagator should be out in your greenhouse or potting shed, not the airing cupboard.

The airing cupboard is fine for making dough rise but not for your propogator ! If you haven't got a greenhouse or shed use a windowsill or a table near a window.