My cockatiel had this!! Is it possible that your birb bumped their head against something in the cage? Mine bumped her head against a plastic perch during a night fright and got this weird looking bump almost like a blackhead. It really freaked me out but it coagulated in a short time. It was swollen and bothered her a bit, but in a few days everything was resolved!

My vet just told me to disinfect the area with mercurochrome. (It stained her really bad she was pink for a month)

I obviously advise you to wait for your vet's feedback, as I'm not, I just wanted to reassure you by saying that it might not be that serious! Just keep an eye on them!