When you are writing a post there is a button on top of the box (a couple of inches above the text box) it reads "choose file" you click on that and then options for ways of posting a pic come up. First get your plant in something with drainage holes or drill a hole in your pot. It would really help to have a pic to see what type of soil you are growing with. Then we could assist you more.

Probably a good idea to dig it out and separate it from the bunch. I've only dealt with mealies and these definitely look like something else. Hope you can find a cure for it, looks like a nice plant. Is it Crassula "Morgan's Beauty"? Could be "Coralita" as well

Any damage that is showing on your plants is mechanical - that means it came to be from shipping and other transport, handling or jostling. It is not the result of diseases or insects nor does it look like any watering issues. In reality, these plants look pretty darn good considering most big box stores do not take the slightest bit of care with their plants (hence the evidence of damage you are seeing). As to fertilizing, you need to identify your plants and then do a bit of research, as not all plants benefit from similar fertilization methods nor do they require it at the same time. Cacti and succulents have different needs than do tropical foliage plants and flowering houseplants (orchids, African violets, flowering gift plants) have yet another set of needs. In general, an all purpose liquid fertilizer used at 1/2 to 1/3 strength with every other watering is appropriate for most tropicals during their primary growing season (March through October) but you can get by with quite a bit less. I'd suggest you post this on the House Plants forum both to get accurate ID's on your plants and their advice as to when, how much and with what to best fertilize what you have.

First, don't do any treatments until you know for sure what it is. Fungal disease should be treated with a fungicide. I had some fungal disease on one of my aloes and treated it with powdered cinnamon (natural fungicide). I wasn't 100% sure what it was, and couldn't get ID here either. What I did helped. I didn't isolate plant and it didn't spread on other plants either. It may depend on what disease it is exactly - I am not sure. Rot usually spreads thru the plant, not really on other plants unless in same pot. It usually happens with overwatering, and culprit may be/often is water retaining soil. Depending how bad it is, it may be good to cut off infected parts, repot, check the roots...