to the forum.
Unfortunately I can only say that those patches will spread, the plant will live but those brown rusty patches will most likely become permanent scars. I have quite a few of plants have those and I don't know what caused them, I suspect it is something to do with water, humidity, air flow and light exposure. I gave up in treating some of the plants and moved them outside full sun full rain, and surprisingly they become healthy, the new growth all looks clean and fine. My brother has one mammilaria that at the end grow new skin from inside and let the brown patches peelled off, that is the only plant that I have seen can fully recovered.
I will suggest you to let the plant has better airflow.
Below are some of my plant that have those, the new growth are all fine.
This gymno just get those in the past 2 months, and the rusty patches are growing/spreading.
