large white spot on neon tetra

Hey guys, Today I discovered that one of my neon tetras has a large white spot. About 2-3mm large diameter just diagonally left, below the dorsal fin.

Any idea what it is? I dont think it's ich because I heard ich's symptoms are lots of white spots....

Also any idea how to cure it and prevent it from spreading in my display tank? I've already quarantined it....

Please help! I don't really know what to do