ANSWER:

The ghusul is invalid and one must repeat it and perform tayammum for not washing that area covered by this layer. 

I got chemical henna applied, not knowing it sets a layer on skin,i found out about it after some days, i had done ghusl to remove major impurity(menses) in those days, is the ghusl and prayers following it not valid meaning i do ghusl again with tayammum? i hope this can be excused and valid due to ignorance.. note: this layer can be felt and be peeled off on some of the parts, others not