something they call Ich, sometimes spelled Ick. There are additives for their tank water that treats it if this is what it is. Various formulas are available at a good pet store. If I recall correctly, it's a type of skin fungus and the best way to treat it is to treat the entire pond or tank and then to do preventive doses, after it's cleared, on an occasional basis. Most formulas will have good suggestions for follow through on the label or inserted packaging.

Yes, it's contagious and will infect all the fish in the tank eventually. Catch it early and be consistent in treating it and you shoulld see some dramatic improvement.

No, you didn't necessarily do anything wrong, assuming you check the ph in the tank and keep it within proper limits. Ich just seems to happen and is often brought home with new additions to the tank. Best argument for isolating new additions temporarily.

Hope this helps.

Callie