I put what i thought was polyurethane now my wood floor has white residue what could be the problem?

I was doing renovation in my room and decided to sand the floor a bit to get rid of some starches and paint it to make it look newer. so after i sanded I put something on my floor I'm not sure what it was it could have been anymore I don't have the can anymore... polyurethane or maybe stainer i didn't wash it off i let it stay until it dried and now months later my floor has white spots on the floor that you can starch off with a finger nail what is this? what did i do wrong? and how can i fix it?