That must have felt horrible, I’m sorry.

There are two options here, broadly speaking. Either you bring it up with this person’s supervisor or - if you feel comfortable - you take that registrar aside and tell them directly ‘you said this thing to me, it was racist even if you weren’t aware, and I’d like your assurances that nothing like that will happen again’, or something along those lines. Either would be entirely justified, and either may be effective (you know better whether this person is capable of being embarrassed into considering their actions and changing them or not), it’s just down to what you prefer. Good luck, whatever you choose. This shit is totally unacceptable: I’m sorry you had to experience it.