I've been noticing white stuff on the part where they took the tooth off. what is it?

Could be: Several things, many normal. Not enough info to really be helpful. But a common finding I have made when I have similar findings from my patients is normal dead tissue. The cells lining your mouth turn over quickly. They are white. When you are not eating much , restrict you oral movements, and you are being careful with a particular site, the white tissue accumulates and becomes visable.

Many things: Could be tissues just healing, food particles or infection if it's associated with pain , dwelling or fever. But if all is good , oral hygiene and post extraction instructions followed then you don't have to worry..