Yes fateh Halab was established by Jeish el Islam leader Zahran Alloush. He was always at odds with Nusra (hes saudi backed).
However, Jeish el Fateh (which did most of the heavy work in the recent push in northern syria), is supported by Turkey and Qatar and Nusra is at the heart and core of it.
Fateh Halab is just a new name for the saudi orbiters there. They didnt merge but the recent push was a sign of coordination and acceptance by Saudi Arabia of Nusras role. The free syrian army turned out to be a joke, Zahran Alloush was stagnating, whats making a difference are the islamists now.
Whether thats temporary or not remains to be seen.
Plus Turkey isnt about to heavily arm the rebels like it did just to hand over the victory to Saudi Arabia or Alloush. They can either find a solution or go head to head like theyre doing in Libya.
The islamists were recently asked to establish "civil law" in a city the rebels occupied, and their reply was "the one who fights the battles is the one who dictactes the rules". And the ones fighting now are Al Qaeda, and theyre openly supported by the opposition camp. ANd now that city is under Sharia law