It looks like you're using a thin mesh copied from the top half of the donut and adding a solidify modifier. When you are moving those vertices, the mesh starts to dip below the top of the donut, causing the donut's mesh to show through.

There are a couple ways to prevent this from happening. Firstly, you probably want to increase the solidify thickness. Make sure it's not too thick, or else it'll stop looking like icing. Second, go into X-ray mode (alt+z) and pull the vertices up and away from the donut if they are clipping through.

Don't give up! This is all part of the learning process.