What eco-friendly substance can I use to write and erase on a wall for UV glowing without detection in daylight?
I think that in a wall you can't write and erase so easily. You can write and then re-paint the surface in this case you can use Zinc White (make sure that what you use is $ZnO$). It was a common white pigment before $TiO_2$ discovery and has a strong fluorescence under UV light so if your wall is white (best $TiO_2$ painted, $CaCO_3$ may have some fluorescence) you will not notice $ZnO$ area in visible light but only under UV light.