What you saw was a tube arcing. I saw it once on a Rogue Cronus Magnum. The tube had a blue arc going up and down it. I replaced the tube and also the fuse that was in line with each tube, then had to rebias.
Did I just ruin my amp
First, this is my first tube amp hence the dumb mistake. Over the past few days my stereo has been making an intermittent high pitch squealing sound which I thought was caused by some ground or feedback issue but now see was a power tube dying. Yesterday my wife said she heard some weird sound coming from the speakers. She called me in and I saw what looked like fire in one of the power tubes. I immediately unplugged everything and that is where things now stand. Help on what to do next would be appreciated. I have a full set of extra power tubes which I was thinking of putting in the amp but I wanted to hear from more experienced people before doing this. Thanks.