@snickerish - I hope I can help!
The fluid mosaic model is a model that shows where the substances such as proteins and lipids within the cell membrane are arranged. The model has now been around for over 30 years.
So I think the fluid mosaic model has to do with selective permeability because they both have to do with the cell's membrane. Selective permeability describes the property of the cell membrane that allows the cell to control molecules moving in and out of the cell via the cell membrane and the fluid mosaic model shows you the arrangement of the substances of the cell membrane.