All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
If you do not know when this white substance in the navel occurred, and it was possible that it could have been there after you had performed Ghusl, then it should be considered as if it occurred after Ghusl and your Ghusl is valid and you are not obliged to remove that substance (until you have to perform another Ghusl). Similarly, if the substance does not prevent the water from reaching the skin, or that according to the custom it is a very tiny thing, then Ghusl is valid. The jurists stated that if there is something that prevents water from reaching the skin and its time was not known and it could have been occurred after purity, then the purity is valid. They also stated that a tiny thing that prevents water from reaching the skin like the dirt in the nails does not constitute a problem (if not removed) wherever it is in the body.
On the other hand, if the substance prevents water from reaching the skin and it could not have occurred after the recent Ghusl, then it is an obligation to wash the navel only with the intention of removing major impurity and it is not an obligation to renew the whole Ghusl. For more benefit, refer to Fatwa 118955. In this case, all the prayers that were performed with that Ghusl must be made up. If one is not sure about the number of those prayers, then he should consider the number he predominantly thinks he performed. Nonetheless, Shaykh Ibn Taymiyyah is of the view that it is not an obligation to make up the prayers in such a case.
Allaah Knows best.