To me it looks like a cultural problem, not a disease and definitely not rust. I see algae on the soil surface. That implies poor conditions. Too wet and possibly insufficiently ventilated.

Yes I noticed the algae and has been putting cinnamon powder on the soil. Should I get a fan as well? The planter is on a utility shelf, there are not too many plants so overcrowding is not a problem. What else do you think I should do?

I live in San Francisco, right now the temperature is between 54 F to 45 F outside. But the mints are indoors on a shelf with a grow light shining on top of them. I don't think light is a problem because the succulent placed next to them is happy and blushing. I will try getting a fan blowing from the ceiling if that helps.

At those temperatures I think it would be better off outdoors. How about potting a couple up, hardening them off and growing them outside? Personally, for environmental and financial reasons, I wouldn't use electricity for lights or fans on hardy plants like peppermint, but that's obviously your choice.

Btw The conditions which favour succulents are entirely different from those enjoyed by peppermint.

I live in an apartment so I can't grow plants outside. The grow lights are LED, they don't cost much money :) I notice some new leaves on my mints, will wait to see if they grow bigger.

OK, I see. In which case just give them as much light as possible, not too much warmth and a good amount of air movement. In the summer pls I get them by an open window would be useful. Do you have a balcony?