I'm hoping that this is the appropriate place to ask this question - I've had this cactus for a long time and it seems to be developing some kind of fungal/mould problem. It lives on my parents' front porch (which faces north) and so gets near full sun, good air flow and some exposure to rain. That said, we have had a wetter than usual year and so I suspect this is probably a factor.
One of the two stems also appears to be turning yellow and looks ill. Historically it has been damaged some years ago when it tipped over and broke the top section off both stems (in both cases they sprouted new growths and continued growing).
Cacti are not my area of expertise, so any advice would be appreciated. Some suggestions I have read are to cut out the infected tissue, or in the worst case cut off the healthy part and try to grow it from that (no idea if that would work this type of cactus, though, as I'm not even sure what type it is).
