Semi transparent pixels on mosaic effect?

Apply an additional adjustment like levels to control transparency.

Mylenium 

If clicking Sharp Colors does not fix the aliasing interpolation, you will need to adjust your artwork size. 

Cropped screenshots don't tell us anything about the comp. You could be looking at the comp magnified at 800%, giving you aliased colors unless your artwork is precisely lined up with the pixel grid. 

In the future, drag your screenshots to the reply field so both you and us can without downloading, and make sure that the comp is selected in the Project Panel so we can see the composition settings and the modified properties of the problem layer are revealed by selecting the layer and pressing the 'u' key twice. You can also drag out the screenshot size so it fills the reply. 

Off:

On:

Use the Drag & drop here to share things like project files that can't be seen in your reply.