I don't think that is a mouse. They don't crap like that. They walk and crap walk and crap so you will often see a trail of shit along the path taken by the mouse. A mouse that has made a home indoors will not not live in the soil (too much work when there are plenty of cozy places to live once inside) though they may dig up the roots to eat them. If you suspect a mouse look along the edges of the room where the wall meets the floor. Mice will run along the edges of a room looking for places to hide. You will find turds there. I do battle with the bastards every year once the weather turns cold. I won early this year!

It sure is odd looking. I'll give you that. Any chance you could have worms in the soil? That almost looks like worm castings.