It will be UV side. I had this with loops left a long time near certain UV lights and some pastel coolants. I will post my own pics ones i get them off my phone. I will say that it is likely not the tubing. Why do i say this? I experienced the same problem with a tube reservoir aswell!
It looks MUCH worse than it is! If you feel it, it isn't slimey or smooth like algae would be, it is rough like a very fine sanding block. Chuck the tubing but if it has got anywhere else (like inside clear plastic reservoir or blocks), a stiff scrubbing brush or a damp cloth and muscle work is enough to get rid of it in combination with some boiled DI water. Doesn't need to be spotless, but a scrubbing brush would probably make it near enough spotless anyway, if you have something to clean.
BTW, i found some UV lights to cause this and not others. Though certain broad spectrum non UV bulbs have an effect if you keep them on close enough and long enough. There are UV lights which emit different wavelengths to what we standardly use to decorate our PCs, but due to the specific use of these and limited production they are difficult to get.
Dont know the source of the green stuff, but if it is the same as mine (99% sure) can rule out algae and definitely link the problem to UV light though. More specifically to UV lights which have their peak intensities around 350nm. My two week tests showed small increase in the stuff with these lights (350nm) and near none with lights that have peaks at 340 and 370. Whether these other lights can cause this problem over a long period of time, probably depends on whether they use a mixed Phosphor for luminescence or a single compound, as a mix would have multiple peaks around 350nm. It could also be that the reaction happens more on high energy light but the 340nm UV light was just not intense enough to have the same effect as my 350nm Club light (very likely as the club light is huge) and lower energy light at the 370nm+ wavelengths, simply cant encourage whatever is happening.
My specialty at university was radiation and light physics but my knowledge on coolant reactions is very limited. I am sure all you watercooling Gurus can add their two cents and a solution wouldnt be too far away.
I have moved the reservoir along so that you can see a clear patch in the green. This is where the black clips were positioned and blocked the UV light from reacting. I imagine the pastel coolant is too opaque to let the light reach the other side of the reservoir
This is just some of the tubing located near the light. The light was removed to perform the tests, but it went alongside the motherboard.
), while these LEDs are are just as bright or as intense as cold cathode, the photons themselves are lower energy.