Do these pictures suggest mold is growing behind walls?
No.   But maybe.   You would be surprised how easy mold grows on the surface of a wall - especially sheetrock.   It certainly looks like there was/is some offset moisture from those pipes which looks like it has dried up.
While there is mold on the wall it isn't even that bad on this side which means it could be worse on the other side but me guesses that the water is from the pipes... not the other side.   So I would just scrape off the mold (sure hit it with bleach or whatever).   This is a good area that makes a point to use an oil based paint.
Probably the best thing to do is clean the area up, get the mold off and then check it every day or two.   There is no reason to open up the wall unless you can verify that the wall has remained dry, on your side and the mold progresses to grow more.