Catalyzes the reduction of malonyl-CoA to malonate semialdehyde, a key step in the 3-hydroxypropanoate and the 3-hydroxypropanoate/4-hydroxybutyrate cycles. 3-oxopropanoate + CoA + NADP(+) = H(+) + malonyl-CoA + NADPH Optimum pH is 7.8, with half-maximal activities at pH 6.5 and 8.5. Optimum temperature is 85 degrees Celsius. Homotetramer. Belongs to the aspartate-semialdehyde dehydrogenase family.