New York (7-1-8) stretched its club-record unbeaten streak to 12 league games. Moralez has a goal or an assist in seven straight games, totaling six goals and eight assists since May 4.
Moralez gave NYC a 1-0 lead in the 58th minute when his deflected shot bounced over goalkeeper Stefan Frei. Medina made it 2-0 in the 77th with an easy redirection for his first goal of the season and Ofori sent in a 35-yard shot 10 minutes later.
NYC has 12 goals in its past three games.
Seattle (8-5-5) has lost four of its past five games. Seattle's best scoring chance came on Jordy Delem's header off the crossbar in the 85th.