Fungus fruiting bodies. You can wash them off with soap and water. Bleach and rinse well.

If it's really stubborn, just recycle the pot.

Thank you! I've removed all visible ones from the rootball of my cactus and planted it in new soil.

Would you recommend treating the soil with anything? Or should it be okay in new soil?

Clean it up (the plant), toss the pot in the recycle bin and repot in new soil mix. Really shouldn't cause any problem. I have them on occasion, too, even with gritty mix.

I forget the name of it, but it doesn't affect the plants at all. It looks nasty, but it isn't nasty.

Were you thinking of the slime mould Myxomycota? (Slime mould is also not really a fungus/mould).

I've seen it (yellowish fuzzy balls) many times before. If removed, I never saw it flourish in gritty mix after that with my plants. My pots are raised on grating, so the bottoms are always dry.

Thanks everyone, I've done all that. I just wanted to check it wasn't something really nasty as i'd never seen it before!