mRNA decapping enzyme that specifically removes the nicotinamide adenine dinucleotide (NAD) cap from a subset of mRNAs by hydrolyzing the diphosphate linkage to produce nicotinamide mononucleotide (NMN) and 5' monophosphate mRNA. The NAD-cap is present at the 5'-end of some RNAs; in contrast to the canonical N7 methylguanosine (m7G) cap, the NAD cap promotes mRNA decay (By similarity). Mediates the hydrolysis of some nucleoside diphosphate derivatives. Has a high affinity for NADPH compared with that for NADH (PubMed:18815383).