predators die out.
prey overpopulates and overconsumes resources
prey dies out
So why do people always look for a direct advantage for the prey? Why couldn't an advantage be that predators see them more readily and keep their populations healthy in regions where overpopulation and resource scarcity would have otherwise wiped out the population?
There would be selection against the prey that were more obvious to predators looking for prey.
This would be a short term immediate response to the different color.
The selection for the unusual color to keep a smaller more manageable population would be something that works over longer time spans.
For this kind of selection to work, the shorter term selection would have to be countered in the short term somehow.
it would require special circumstances for selection to work that way.
The general idea is that evolution flows through time by paths where each step has some adaptive advantage.
Evolution does think ahead, but sometimes situations can select for things for other reasons that can be adaptive when presented with a new situation.
It should also be pointed out in this case that the fluorescence probably doesn't negatively impact the platypus's avoidance of predators because it won't be fluorescing at night when the fluorescence could be seen.
Solar UV would excite the fluorescent molecules (in nature) only during the day when UV is present in sunlight.
The fluorescent molecules may have been selected for as a sunscreen (absorbing UV, releasing energy as a rather dim green light during the day when it will not add much to visibility), if UV damage to palatypus tissues was important.
Or it (the evolution of the fluorescent molecule) could just some random thing that evolved because there was no significant selection against it.