In a mediation when they had already agreed on visitation and one party already signed the papers. Can the other party take back what they agreed to?

Maybe.  If mediation has been concluded and the parenting plan has been sent to the court, there is usually a "return hearing" -- if one of the parties wants to back out of the deal made, they will usually be asked why, and then it is up to the judge as to what to do about it.