The Flash seems to be hinting quite heavily that Mystery Girl is Barry and Iris’ daughter.
Mystery Girl’s lightning may be the clue as to how exactly she’s related to Barry, if that is the case.
It would certainly explain why Mystery Girl fled as soon as she spotted Iris.
Barry’s lightning is and has always been gold, and Iris’ lightning was purple during her temporary stint as the speedster savior of Central City earlier in Season 4.