What eco-friendly substance can I use to write and erase on a wall for UV glowing without detection in daylight? I rent an office and want an alternative to a whiteboard and continual dry-erase marker purchases. 

I can shut out the light and use the surface under UV lamps only, that's fine.

Commonly listed organic glowing elements are blood, urine and semen. None of which, I think would be untraceable by eyes (or nose) during the day!

The surface I would like to use contains typical white interior paint, applied last year.
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.