If your Center has enabled the setting for guardians to be able to chat amongst each other, any guardian in your center could start a chat message with you through the Playground app.
If you'd like to turn off the ability for other guardians to be able to chat you, open the Playground app and tap the menu icon in the top left of your screen. Tap "My Account". Toggle the first "Discoverable" setting to the left so it is grayed out. This turns the setting off so that guardians cannot find or chat message you.