I have mild developmental prosopagnosia. It is very difficult for me to recognize people, particularly if I have not seen them in a while, or if they're in a context I'm not expecting.
If I have enough time (sometimes up to 10 minutes), I can usually figure out who they are, although I rely heavily on contextual cues for this (hair; clothing style; distinguishing marks such as tattoos, scars, or skin blemishes; body type; etc.).
On occasion, I will run into someone who appears to recognize me, but I have no clue who they are. Very rarely, it turns out they mistook me for someone else. Much more frequently I know them, but it is out of context and I don't recognize them.
When this happens, I usually try to act like I recognize them, but I fear it is not always convincing.
So, is there a tactful way to explain that I have trouble recognizing faces without seeming rude, or weird?