You are access protected properties. Properties that are not published are not officially supported. The way the TTMSFMXEdit currently works does not support the Prefix and Suffix properties yet.
We are aware of the fact that when creating a descendant class you can access protected properties, but they are currently not supported. We have already added this on our todolist for further investigation. The money editor type automatically adds separators when typing a number, but we agree the prefix and suffix are properties that come in handy in money editing situation. As soon as time permits, these functionality will be exposed.