Catalyzes the hydrolysis of methenyl-H(4)MPT(+) to 5-formyl-H(4)MPT. 5,10-methenyl-5,6,7,8-tetrahydromethanopterin + H2O = H(+) + N(5)-formyl-5,6,7,8-tetrahydromethanopterin One-carbon metabolism; formaldehyde degradation; formate from formaldehyde (H(4)MPT route): step 3/5. Belongs to the MCH family.