Flower < 5mm diameter
Towards base of the plant, leaves are opposite and laterally compressed
EDIT: leaves of plant attached - unable to get a clear shot of the leaves without picking the plant
Mitrasacme polymorpha leaves are 2-15 mm long and 1-6 mm wide. There are none in your photo. The flower is growing up through another plant whose stem and leaves are visible in your photo. I have no idea what the other plant is.