Could be a mold or fungus. Make sure the soil dries out before watering, and keep the plant somewhere with good air circulation.

Thanks! That's what I was thinking. The soil never really dries out though cus I ran out of succulent soil and mixed it with regular soil. I guess I shouldn't have done that.

Probably too much organics on top of overwatering/water retaining soil. I would replace the soil ASAP - it is as easy as getting bag of perlite and mix it 1:1 with C&S soil. And making sure container has drainage hole(s) too.

Great advice! I will change it right away.