Strange yellow thing that appeared on top of blood clot after wisdom teeth removal. what could this be?

Foreign object?: One possibility is a piece of food such as a kernel of corn fragment has become embedded in the top of the clot. If the extraction was performed more than 3 or 4 days ago, it should be safe to carefully lift the fragment off with tweezers or go back to let the dentist look at it if you aren't comfortable with tweezers or if the extraction was more recent.

Normal: This is probably normal because the blood clot changes color from red to brown to yellow to white. If you have concerns, see your dentist.