This is a refusal of entry stamp. Unless you have also been banned (which is unlikely unless deception was involved), a refusal doesn't prevent you from reentering the Schengen area and won't be recorded in the SIS. You might reasonably expect additional scrutiny next time you cross an EU or Schengen border (or even possibly other countries' borders) so bring documentation to support all your claims. The things the Dutch border guard mentioned (joint bank account, rental agreement) would certainly seem useful if you hope to be treated as a member of an EU citizen's family.
From our discussion, it appears that the reason to refuse entry was
Wordt beschouwd als een gevaar voor de openbare orde en de binnenlandse veiligheid, de volksgezondheid of de internationale betrekkingen van een van de lidstaten van de Europese Unie.
It's one of the standard reasons to refuse entry in the Schengen area. Prior to the Covid-19 pandemic, it wasn't very common but it does mention public health (volksgezondheid). Since the border guard wasn't satisfied that your relationship with a German citizen was enough to consider you a member of the family of an EU citizen, they have to apply the regular rules. And it is currently forbidden to enter the Netherlands from outside the EEA except for trips deemed essential by the government. It also means that reentering shouldn't be a problem at all once those restrictions are lifted (and you can easily explain that if you are asked about it in the future).
In any case, there is no way and no need to remove the stamp as you are still legally free to enter the Schengen area and border guards will not automatically refuse entry. Instead, they are supposed to determine whether your situation changed and you now fulfill the conditions for entry. You do need to figure out a good plan to secure residence somewhere for the next 6 to 12 months as travel is very restricted at the moment (and for good reason).