Green’s Dictionary of Slang

an extremist in the cause of environmentalism.

1973		Atlantic Monthly Nov. 24: Sierra Club members, taunted as ‘ecofreaks’.
1976		N. Thornburg Cutter and Bone (2001) 88: Had Bone heard the one about the guy being an ecofreak, that he stashed the girl because he was against littering?
1986		J. Ellroy Silent Terror 109: [T]he ‘Ecofreaks’ had put the kibosh on Big Sol's ‘Singles Paradise’ tract.