The fancied red-and-gold brigade were caught unaware early in the sixth minute when Gim Dae Han put SunMoon in the lead as the young outfit acclimatised themselves wonderfully to take the first-half honours.
At the same time, East Bengal were let down by their profligacy with Shylo Malswam Tulunga missing as many as two sitters, while Moga and Edeh Chidi up front were outplayed by their opponents.
Midway into the first-half, East Bengal had their best chance when Tulunga's left footer from six yards went over the crosspiece.
But East Bengal were quick to regroup at the break while the Busan outfit showed some signs of lethargy, something that acted in favour of the local outfit.
In a 77th minute move, Lalrindika Ralte's left-sided cross was missed by SunMoon goalkeeper and the ball went to Chidi.
The Nigerian's shot from the right took a deflection from the Korean goalkeeper's hand before Moga gave a subtle finish to save East Bengal the blushes.