In this speech, President Truman was announcing the so-called Truman Doctrine. This doctrine stated, in essence, that the United States would support any nation that was in danger of being taken over by communist forces. But Truman did not specifically talk about a threat from communism. He only used the word "communist" or any variation of it once in the speech.
Instead, Truman spoke of how Greece was being endangered by "a militant minority, exploiting human want and misery." It was clear from the context that Truman meant that this militant minority was communists. He later went on to confirm this by saying that the people who were engaging in "terrorist activities" were "led by Communists."