After eye surgery, what is the white booger that builds up at the corner of your eye?

Prednisolone. Usually this is the white drop (prednisolone/predforte) that leaves some residue behind. This is ok and will go away when you finish your drops.
Accumulation of medi. One of the most common eye drop given after eye surgery is a steroid drop called prednisilone acetate. This drop has a whitish participate which can often accumulate on the corner of the eye.