I can't see it from here, but if it is just surface rust then it might not be taking away from the strength. If you hit it with some rough sanpaper is there shiny metal just beneath the surface rust or does the rust go in a ways? If it is just on the surface I would sand all the rust off, paint with a rusty metal primer and then maybe even a top coat of rustoleum or sim. If you are left with ANY doubt as to the strength then replace or reinforce it. If there is a joint in the girder above it then a replacement would have to be exactly in the same place and both sides would have to be temporarily posted up so that the floor won't fall on you and kill you when you take the rotten one out. The rust might be from a moisture issue. You might have moisture wicking up from under your house. Anything that is left in direct contact with a concrete pad or slab faces the prospect of being damaged by moisture. The only way to protect lallys it to paint them as well as possible.