This odd rough stalk in the center of the image has been growing sideways for a few weeks now, what is it?
That's an aerial root. Monsteras are climbers; they grow aerial roots to grab onto trees or other structures to support themselves. It's a normal thing for a Monstera plant to do, and generally means it's happy and growing as it's supposed to.
If you hate the way it looks you can trim it off. But you don't need to; it's perfectly fine for the plant to just let the aerial roots grow. If you like, you can provide your Monstera with a pole or trellis to grab onto; that will encourage the aerial roots to grow toward the support rather than all over the place.
There's another aerial root starting to grow just above the long one: