MOSI (Master Out Slave In) is an input on the slave, so it is always high impedance. The slave won't be able to tell if the MOSI line is high impedance at the master, but then it doesn't need to. If the slave is selected (SS low) then it expects to see 1's and 0's on MOSI. If it is deselected then it doesn't care.
The reason for having high impedance on MISO (Master In Slave Out) in the slave is so you can have several slaves on the same SPI bus. When SS (Slave Select) is high the slave makes its MISO output high impedance, so it won't collide with another slave which might be selected.