You havent specified any image for marker..So add it like this..
map.addMarker(new MarkerOptions().position(pos).title("MY POSITION").snippet("GPS").icon(BitmapDescriptorFactory.fromResource(R.drawable.yourmarkericon)));
and
Remove your marker from onCreate()
as it is called only once at the time of activity creation..
Add your marker inside onLocationChanged()
or add marker in a function and call that function inside onLocationChanged()