Please try to upload photos or videos of the affected area Nazareth .
Maybe its just hole in the head what your discus is experiencing . The worm looking pieces might just be skin detachments hanging from the head.
The fact that it respond so quick on Metronidazole also drives me to belive that its HITH related.
I would try another treat with Metronidazole but this time perform the treatment to at least 10 days . Raise the temp. To 92 F for the length of the treatment and change a lot of water daily (70 % )prior every redosing .
But please upload pics before you start any treatment for us to try and confirm anything solid .