I would call that "gauze" or "gauzy". If 100% cotton, that's what I would use for bandages or curtains!. I would not use that for foundation piecing.
"Muslin" - is sort of like quilting cotton - but usually it's a solid color - white, cream (natural), or light tan in color. In fact, a lot of people have used the white or cream as the alternate blocks when making a top or back of a quilt.