Certified GKH Gardening Expert
That looks like a response to overly wet soil. They will exude a sort of "sap". You can prune these leaves off, as they will remain unsightly.
I would hold off on the watering until the soil is dry, almost, halfway down the pot. They are very drought tolerant. Try and increase the temperature around them, also, if possible. They tend to do better when it is hot.
Here is an article that will help you care for sweet potatoes: