Catalyzes the reversible reduction of methylene-H(4)MPT to methyl-H(4)MPT. 5-methyl-5,6,7,8-tetrahydromethanopterin + H(+) + oxidized coenzyme F420-(gamma-L-Glu)(n) = 5,10-methylenetetrahydromethanopterin + reduced coenzyme F420-(gamma-L-Glu)(n) One-carbon metabolism; methanogenesis from CO(2); methyl-coenzyme M from 5,10-methylene-5,6,7,8-tetrahydromethanopterin: step 1/2. Belongs to the mer family.