No, it isn’t. You may contact any school by phone, mail or in person whether or not it lists a current transfer on the Open Market website. The UFT/DOE collective-bargaining agreement gives members the right to apply to as many schools as they wish during the Open Market period and to accept a transfer before the period ends. (After that date, your principal must release you.) The Open Market Transfer website is a tool to identify vacancies, but other means of contact are permissible.
See more about Open Market and transfers.