25GbE port on H610 is a Mellanox ConnectX-4 Lx ethernet adapter. According to Mellanox community article, the FEC mode is auto-negotiate by default. If the other side supports FEC, FEC is then enabled. If not, FEC is then disabled.
FEC modes supported of Mellanox ConnectX-4 Lx ethernet adapter are AU(Auto)/NF(No-FEC)/FC(FireCode FEC)/ RS(RS-FEC)