Thanks so much for all your replies! Sad to hear it looks like scaly face mites, but if I am reading the articles, it's only started showing up fairly recently and should still be well treatable
I do think I'll need to treat my 'tiels for it as well. I didn't notice until yesterday and didn't take any precautions when handling the budgie or 'tiels before.
I even still have a dropper at home which contains ivermectine for when I had parrotlets that had feathermites and according to the vet I contacted, possibly airway mites.
I'll give my vet a call. They're not specialized in avian care, I think but are very knowing of it
I think vets specialized in avian care are sparce in the Netherlands.... The only place I know for sure has avian specialists is the University for veterinarians in Utrecht, and they only take special cases I think, and mostly large parrots and birds of prey and such!