Just clarifying. Has the mould formed on the back of the LCD window glass or on the inside of the LCD display itself?
If it is on the back of the LCD window glass then here is a link to the Ifixit guide on how to remove the back casing of your camera. Hopefully by using this guide you will be able to gain access to the window glass to clean it.
Canon EOS Rebel XTi Back Casing Replacement
If the mould is on the window glass and cleaning does not remove it for some reason or other then here is a link for a replacement part.
If the mould turns out to be in the LCD itself, which would be unusual as it is supposed to be a sealed unit, then the only thing to do then is to replace the LCD unit.
Here is a link to just one supplier of the part. This is just shown to give you an idea of the price of the unit. Search online if you need or want to get the part.
Hopefully this is of some help.