How do you remove mold from the back of oil paintings on canvas?

You really probably cannot.Keep them in a very dry,cool place.No direct sunlight that fades the picture,If it is only the back of the canvas,Very carefully you can try a very very light application of lysol..(or very mist spray mold killer,let dry,spray again lightly ,let dry..until you are pretty sure that has illimated most of the alive active mold(Do not over due, use very quick even small amounts,you do not want to saturate or retard the canvas,give great amounts of time to dry.(As with anything you can always add more but you cannot take this away once its done)If these are "oil" paintings it should be fine.You do not need an expensive art restorist.The damage is already done by moisture.You cannot remove the mold discoloration.If you are satisfied or feel that was careful enough and enough . Then I would buy a "clear flat matte finish" sealant spray enamel.there are inexpensive ones and expensive ones ,You can very carefully and lightly "seal"The paintings back with this. "Do not saturate"Spray very short light even application. repeat when very dry do not be eager,but be short and quick when applying it,shake the can every which way longer then it advises.Spray once across fast ,do not be tempted to spray the same area again.You are not suppost to see shiny wet transparent matte enamel finish.let the whole back of the canvas slowly absorb very small amounts.This will stop the mold and the moisture from continueing to destry the oil painting. You notice the painting itself has none (most likely). Thats because the paint already is a self sealant which the average mold cannot grow well on.(Of course that depends on what kind of mold it is)..I JUST NOTICED that you wrote the mold is showing through on the front. I recommend <many short quick aplications of lysol the painting will absorb this to the front .This will not harm your picture don't listen to number one..This will take atleast a week .You wan't to spend 5.00 or 500$ or more.

I have seen art restorers use a sponge and very diluted bleach and water solution