Moldy back

I had that problem on one of my own big paintings, and I used dilute bleach with a sponge on the back of the canvas. I can't remember if I also rinsed that off with fresh water afterwards.

However, I'm not necessarily recommending that, as I'm not an expert on "restoration".

Putting water to a canvas can be ok, a way to "tighten" the canvas, get out what we call "points" that are what you see when a canvas is dented. But I once gently washed one of the paintings my parents had and that was a terrible mistake, as the painter had not used any gesso to prepare the canvas, and the thing just fell apart on me.

So, let's wait and see if anyone else has ideas.

I read the following advice somewhere for the removal of mold on oil paintings.

Use a solution of one part household bleach and 20 parts cold water, wipe the area with this - but DO NOT soak the canvas - then rinse off with clear water. Repeat if necessary. Be sure not to let the solution wet the canvas, you're only removing the mold from the surface. If the canvas becomes wet from rinsing, let it dry completely before repeating.

intrepid has it right