I need advice caring for my hanging cacti. I think it's rhipsalis lepismium bolivianum? It's started to develop black patches along its limbs. Also some weird spots. And some white muck gathering, which I guess is mealy bugs – I've just ordered some SB Plant Invigorator and Bug Killer for that. Can anybody give me more advice? Anybody with experience of this? I haven't been watering much during the winter, as with other cacti; it's in a fairly well-lit spot, a couple of metres away from a south facing window – and was doing well until winter set in proper.
I'm more familiar with desert cactus, but I'll take a stab.
First, if it does have mealies, segregate if from all other plants until the mealie situation is resolved. Unless you want to treat mealies on all your plants.
Next, I would cut off all dead or soft-looking pieces. I would remove any dead organic material from the top of the pot. General clean-up so you can get a sense of the extent of your issues. When cutting, check the remaining stem, if it has a brown spot in the middle cut further back on the stem. You want to get all rotten material.
Then, you can pick the mealies off with tweezers then dab the spots with 70% or less isopropyl alchohol on a q-tip or similar. If they are wide-spread chemicals may be required.
It sounds like all is well besides this new rot and possible mealies. You may have root mealies in which case you'll need to repot the plant, treating the mealies.
Have you ever fertilized this plant? The SB Invigorator isn't a fertilizer, it appears to be a sort of supplement.
Good luck! It looks like you've got plenty of healthy material left.