What’s the difference between the terms Main Effort and Main Element?

Main Element: Primary unit
Main Effort: Primary goal the unit is working towards

main effort is what is happening. main element is who is doing it.

1LT(P) Alexander W. what is the context?

The Main Effort and Supporting Effort, and Reserve are designated by the Commander, usually at the BN and above while n the offensive. The Main Effort is allocated more resources, or if there is a constraint or shortfall, the ME is spared, while the SE takes it in the shorts. The ME is the Main attacking element to take the OBJ, while the SE (may be multiple) launch supporting attacks against secondary objectives or isolating the main objective from reinforcement/escape. The Reserve is committed on a carefully managed criteria, usually at a decisive point in the battle to reinforce success or serve as a pursuit force to maintain contact and increase pressure on the enemy.

If I remember my 7-8 and 7-10 a Main element refers to a portion of a platoon or a Company or even a Battalion that is flanked by flank security, has a lead element of about 1/3 of the combat power and a small rear security element. You have units in the ME, SE and Reserve all configured this way as they move.