This is the song. The word is translated by online lyrics, RAE and wordreference.com as molasses. But it doesn't make sense to me in the context.
Somos la melaza que ríe
Somos la melaza que llora
Somos la melaza que ama
Suggested tag: metaphor.
Melaza is a byproduct of the production of sugar and panela (a.k.a piloncillo) and it's dark brown and very sweet. The song you mention is really called "Las caras lindas" and it's interpreted by the great Ismael Rivera (himself an afro-descendant).
The song talks about how beautiful black people are, how loving, etc. I think using melaza as a metaphor of black people makes a lot of sense. Melaza is the same color as afro-descendant people and it's also well known that Africans were brought to America to work on sugar cane plantations, in Cuba, Colombia and I am sure other countries, it's still a job usually performed by Afro-descendants.
In conclusion, "la melaza" is referring to black people.