I just called about my plant, I have attached some images of the moldy
looking stuff so you can further diagnose,
It looks like your Opuntia sublata has a very bad case of Mealy Bugs! Under all that white “mold” should be nasty sap sucking vampire bugs, they coat themselves with wax to keep predators from eating them…
I recommend you spray the white masses off with a good strong jet of water from the hose (support the cactus branches with a stick while you do, so they don’t break). Then spray the plant with a good coating of 1% Neem Oil solution to kill off the eggs and any you miss with the jet of water. You may also need to drench the soil with the Neem, as they will also live in the soil and feed on the roots as well.