Looks like yolk to me as well--are you sure this baby didn't have an umbilicus still partially attached? Perhaps it came from that.
Digested yolk is often yellowish too, and the first defecation can look a bit funny, just as with any infant.
In any case, I wouldn't worry about it unless the snake shows some other sign of ill health, fails to feed, etc. It looks healthy enough in the photo (and feisty).
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