Is it obligatory to remove the transparent, oily pimples that form on the back during ghusl from janabah, and to wash what is beneath them, and are they considered a barrier preventing water from reaching the skin?

It is not obligatory to remove pimples emerging from the body, as they are not considered a barrier preventing water from reaching the skin. The same applies to dirt and fats secreted by the body, as long as they originate from the body itself.