Was its' environment suddenly changed? Could be sunburn on older leaves not used to so much sun (which I'm assuming since it's paired with Pelargonium.)

Mine did that in one pot last year where runoff from the roof hit the pot. I chalked it up to too much tannin from oak leaves in the gutter.

The very light colored leaved strains are derived from more deeply colored strains and sometimes the light leaves will 'revert' a bit by showing color if they see enough sunlight to initiate the pigmentation , which does act as a protection from the sunlight...

Depending on your point of view , the bit of added blush can still look nice...

It looks like sunburn to me. The sun has been really intense lately in Delaware. Has it been scorching hot where you live?