Hi Trang, it's hard to say without looking at your starter, but if you're due for a feeding try to scoop off/pour out the questionable part on top and feed the healthier looking starter below. Hopefully as your starter progresses you won't see a return of the mysterious yellow spots. If you were seeing pink or orange I would be more concerned, but this might well be something harmless that will be sorted out as the starter establishes itself. However, I would avoid using the discard until the starter smells and looks a little more promising (active bubbling, a pleasant aroma). 

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