This is the white disease for tetra. Normally a tetra attcked by this white spot. It is hard to cure. If it died, othe tetra will eat the body and they will infected by this disease also. And then end up all yr tetra died.

So, once you saw the tetra with white spot. quarantine it in another tank. And there is a medicine to cure this disease from Sera. But i forgotten the name. May be you can goto lfs and ask for it.