Hard to see from the picture, but some cacti can secrete nectar from small areas near their spines in addition to what gets produced in the flower. I do not know if this is the case for this cactus, but if the clear spots dry out to be the white spots then that could be it. They do this to attract more insects which would increase the chance of getting the flowers pollinated.
Alternatively, the clear drops could be attracting scale insects that might make the white spots as they settle in to protect themselves, but I doubt that any of those that like to be on cacti would naturally occur where you are.