That could be a number of things but more than likely it is mold or a spider nest. Prune that part of the plant if possible.

Being that you have soil from outdoors mixed in with your store bought soil, it would probably be a good idea to feed them a low dose of nutes. I would reccomend 1/4 of whatever the nutes reccomend. It really is a toss up though since you used mix soils. Jus give them a lot of light and not too much water and they should be fine after you get rid of the pest problem.