Usually "pop eye/cloudy eye" isn't contagious alone, it is usually a sign of other diseases to follow so watch out. I had a tetra develope this, removed and treated it, fine in a few days. Then the whole tank had an ick outbreak, sigh totally my fault let the nitrate levels get to 40ppm. Most of them survived.
So sorry to hear about the betta takeing a leap! :'(