Gidday Ed, Looks like some of the bugs are scale. Either way, it looks like natureal or hand methods might not be enough. I'd water the plant with a mix of Rogor. This systemic should do the trick. Also another good thing I do, is when I repot my plants, I include some of David Gray & Co "Systemic Insecticide Granules". This systemic insecticide is granulated and is good in the mix and gives a few months protection.

First your pots are too tall. You need shorter pots. You also need to use proper cactus potting soil. Read up on cactus care. You are having a drainage problem. If you are watering your plants more than once a month after October it's too much. If you are in a Northern Climate I would look into a Climate Lamp. Your photo shows you just aren't getting enough light. If you have a southern facing window that would be best. Get them outside as early in the spring as you can.

Your cactus are super etiolated. The new growth is reachin for that sun;)! If this were mine I’d cut the etiolated pieces off with a clean sharp knife, and begin to acclimate the cactus to stronger sun. Not sure if you know this, but those little white fuzzies are called glochids. Protect yourself from the, because they will get stuck in your skin, clothes, and everywhere else. Glochids are VERY irritating.

That’s rot. You could twist off all the new growth thats growing from the rotting pads, and start them over. Opuntia are really quick to throw out roots. Just repot like normal, but keep the cuttings dry until you get roots. Keep away from strong sun till roots form. How’s the weather where you’re at? Is it still warm or starting to cool down?