It sounds like that is what your Clown had. Freshwater dips followed by Formalin treatment is the only cure. I'm not sure if every fish can get this, but almost all can. Clowns just seem to be the most common...Mot sure why though.
if you are in no hurry, your best bet is to leave the tank fallow for 4-6 weeks. That should let what is in your tank die off due to no host.
Keep in mind that any LFS
can sell fish with a variety of possible ailments. They've usually got whatever it is before they arrive, but they can certainly pick it up there as well.
Damsels are aggressive and a lot of people wish they had never got them and end up removing them. I would not get them because of that.