Hi Meduzz , i think i know what it is
Nothing to worry about mine have them to , those brown spots that lays 1 or 0,5 milimeters on the surface of the cacti and its hard like the rest of the plant , those are ancient spots .
It means your cacti is aged and gets some of those brown spots . i dont know why but it is noting serious or to worry about its natural . ( you dont need to cut those away )
I've orderd mine on internet , a cutting i've placed in a pot .. waited 2 a 3 mounths before watering , to make sure the cacti had made some roots and then i've given him some water , depends on the season .
Winters i gife just enough water to prevent drie out off the roots , Summers i give it quit more water , anyway ... every time i gave it water the spots became bigger , now it seems to be stabilazed , the spots dont become any bigger ... its normal it was an old cutting , i dont know about your plants background , but if its the same as above you dont have to worry .
just dont over water the plant its very important , always wait til the soil is completly dry , beter dry for a cacti then over waterd
Good luck , Soulflower 8)