The compost atop my seedling pots are mouldy

[NemophilistRebel]

I have just bought a little propagation tray for seedlings to grow tomatoes and pumpkins ready to plant out when it’s warmer

I checked on them today and a few of the pots have some white fluffy mould growing atop the soil

Am I doing this wrong?

This sounds like lack of ventilation and overwatering. If the propagation does not have inbuilt venting, the just put something between the tray and the lid to keep the air flowing