That is the entry point for plankton to enter into her brain to take control.
Does her face hurt?
You beat me to it. I was going to say it’s where the aliens planted the control module when they removed Hillary’s brain.
Speaking of Hillary’s appearance, the strange tunics she has been wearing are not normal attire. As someone with experience attempting to hide flaws (we won’t go there), those extra-long, boxy, stiff tunic tops are bizarre. Is she hiding something under there? Some kind of machine that keeps her pumped with medication? All I know is, those tops go way beyond what someone needs to hide a big butt and hips.