Eeuugh

My lemon tree is suffering from mould on the soil, and I can't work out what the problem is. I have looked on the internet for possible solutions/causes and only found something for sooty mould which occurs on the leaves.

I am wondering whether my boat is simply too cool for it since the temperature has reduced drastically since a few weeks ago.

If anyone reading this has any top tips please do tell me! The tree has new leaves, and did flower and also has one fruit hanging on from the several that were on there and have since dropped and been guzzled.