I suggest that it's possible your wall was evenly dirty and now that part is clean. You could try washing the whole wall.
Some all purpose cleaners do change the color of the paint. It's too late for the magic eraser now. The damage has been done, assuming that the milk washed off.
I've used the magic eraser on my daughter's aparment's white walls and where I've cleaned hasn't shown once the wall was completely dry. I did end up washing the whole wall as far up as I could reach so that the amount of dirty film was the same. I just used a rag and water for the unstained part of the wall.
I recommend only using the magic eraser for stains. Anything that will wipe off with plain water or mild soap and water is best cleaned in that way. The magic eraser does take off very small amount of the paint. When the wall is shiny you may end up with a duller section. Always test before using any cleaner.
I had an all purpose cleaner that unbeknownst to me contained bleach. Using it left bleached spots on my clothes.
By the way, some paints are not washable and will wipe off if wet enough.