Buttercream turned gray overnight! Why?!

Hi everyone! I’m a novice baker and made this cake for piping practice and also to try sugargeeks easy buttercream recipe, which is kind of a shortcut Swiss merengue recipe using egg whites and powdered sugar. Yesterday when I made it it was a pretty mauve color - I left it in the cake stand overnight and now it’s gray! You can see in the 2nd picture where I smudged it and the pink shows through underneath. Any idea why? Did I do something wrong? It tasted great so I have no idea what happened!