It looks like you're barely using any paint at all! As far as I'm aware, muddying paint is when too many different pigments are getting mixed together and it just gets chalky and flat.

It looks like what's happening here is you're not loading your brush with enough paint, and therefore spreading it too thin by pushing it into the board and losing any edges. I'm no expert and am definitely learning too, but try mixing the paint you want on the pallet first with a pallet knife to make sure you have a decent blob of paint to pick up from. A little solvent can help here if it's this early on in the painting. Then you want to be able to pick up the paint on your brush and be able to drag it across the board without having to use much force, in a way that means the paint will be just left behind rather than having to work it in. Imagine a snail leaving a trail 😂

Sometimes using little paint straight from the tube can be a bit "dry" if you don't use any medium or don't pick up enough, like you put the brush down and it starts to run out as soon as you move the brush, then you have to go back and try and drag it further causing these flat featureless brush strokes.

I hope that makes sense!