What is the ruling on skin that can be rubbed off from some areas of the body during ghusl (such as the upper back, shoulders, and testicles)? Does it constitute a barrier? And does the presence of a greasy texture or trace on the organs of wudu’, without a physical mass, constitute a barrier?

Frozen sweat on the skin and oily substances are not considered barriers that prevent water from reaching the skin. Re-performing ablution (wudu) or a ritual bath (ghusl (definition)) is not obligatory if there is something adhering to the body that prevents water from reaching it, as long as it originates from the body itself, like frozen sweat, in contrast to what originates from dust or similar substances.