That’s not rotting, that’s an anal fin someone has cut off with scissors to remove a section that may have been damaged previously.
Green meds like that usually indicate malachite green ( don’t know for sure that’s what it is yet) which quickly and easily dies the water. You should also see it fade in a day or so.
A shop keeper should give more information on diet, health, water, etc. etc. sounds like this friend of a friend shouldn’t be used again and would not be high on my list.
It’s good of you to take on the fish but you need to learn more about keeping fish, the ones you have and the ones you want, before you get them going forewords and use this as your opportunity.
And of course, welcome aboard, you will find lots of good info here and people willing to help.