Is it safe to use the 3.3 volt supply on my arduino mega board to power my NodeMcu???
The Mega has a 3.3volt/150mA supply, so in theory, yes.
Connecing the 5volt pin of the Mega to the 5volt (V-in) pin of the NodeMCU should be safer though. But why do you want to connect two processors together (with all the issues that come with it).
Rule #1 a power supply the Arduino is Not. Seperate power supplies is the best.
A NodeMCU draws an average of about 100mA.
The Mega draws about 70mA.
It perfectly fine to steal 100mA from the Mega's 5volt pin on USB power.
Question is what else is connected to the boards, and why the heck two processors.