Context. MWC 758 is a young star surrounded by a transitional disk. The disk shows an inner cavity and spiral arms that could be caused by the presence of protoplanets. Recently, a protoplanet candidate has been detected around MWC758 through high-resolution L0-band observations. The candidate is located inside the disk cavity at a separation of _111 mas from the central star, and at an average position angle of _165.5_. Aims. We aim at detecting accreting protoplanet candidates within the disk of MWC 758 through angular spectral differential imaging (ASDI) observations in the optical regime. In particular, we explore the emission at the position of the detected planet candidate. Methods. We have performed simultaneous adaptive optics observations in the H_ line and the adjacent continuum using SPHERE/ZIMPOL at the Very Large Telescope (VLT). Results. The data analysis does not reveal any H_ signal around the target. The derived contrast curve in the B_Ha filter allows us to derive a 5_ upper limit of _7.6 mag at 111 mas, the separation of the previously detected planet candidate. This contrast translates into a H_ line luminosity of LH_ . 5_10