Question about this strange soil and curl grubs.

Hi Gardeners!

I’m located in Sydney and have been repotting some of my plants (mainly cacti). I noticed this strange soil some months back that would collect on the ground underneath a couple of pots…

Today I repotted two cacti and noticed the soil again, then upon digging a small crawly bug came out of the pot and I noticed maybe 8 of these curls grubs inside (photo attached from Google similar to how they look).

What on EARTH is the weird rocky soil that these grubs are obviously causing? And what’s the best way to go about this?

Thanks!