Just an educated guess but it looks like mold. Your pot is much too big for your leaf and it may be staying too wet. What kind of soil are you using ? I would re-pot using a sterile soil mix.
Linda

I side with Linda - it is a some kind of harmless fungal growth feeding on the organic part of the soil. If you have wood chips in your soil mix - that can be the food source for this fungi.
It is unsightly - that's about it.
You can scrape the top of the soil off .

The problem is with your leaf - the brown spot can spread and kill the leaf before you get the babies. If the leaf is something you cannot replace - you can either cut the rotten spot out - or trim the leaf under the rotten spot - and dust cinnamon on the cut.

And Linda is correct again regarding the size of the pot = with fungus and surgery = you can as well move the leaf into the smaller container - and make sure the soil has plenty of perlite - so the roots will breathe. You can take the plastic solo cup = and lop the top half off. In my experience they grow better if the soil is to to the top of the container, not down on the bottom of the pot - I think the soil gets aerated better - and a leaf gets more light.

Good Luck

Irina

Irina,
Would you cut the leaf straight across or in a v-shape to preserve some of the veins?
Linda

Linda - I think you are absolutely right - V-cut would be much better.
I.