Fungus generally need moist conditions to thrive as well as decomposing organic matter......not a common situation with most cactus plantings :-) And the vast majority of them tend to be beneficial or at least benign with regards to the soil and the plants growing in it. It may be that what you are seeing is just an accumulated residue from fertilizers (do you fertilize much?) or hard water build-up but could be some sort of fungal manifestation as well. Just scratching or disturbing the soil surface may disburse it but flushing the container with clean, filtered water and then allowing it to dry can help.

Without some sort of evidence to confirm, I doubt you have too much to worry about.

Impossible to tell with info given.

Dan