What is a female supposed to do if she finds a little sign of mascara after ghusl (could be like minute) when she rubs the eye? The ghusl taken before becomes void or its ok to disregard it?

If you see the mascara in your eyes after ghusl and it is a barrier to water reaching that area, that is, it has substance and is not just colour, then just clean that area and wash it (for ghusl) and then wash the body again.