If your server's visibility in Multicraft is set to default, this allows for just about anyone who has your panel's URL to talk in your console chat.
No need to worry about your security being breached or the safety of your server, this merely means that someone can talk in your console chat and that is it.
In order to fix this, you simply log in to Multicraft, look for server visibility under Show Permissions and set Server Visibility to Owner Only (or Users with Roles only if there are other Multicraft users that have privileges to your server).
Here is a visual:
