The 'star' is shown due to a priority setting in your notifications set up. It could've been 'set' by accident and isn't usually a big deal in terms of operation.
However, to remove, you'll have to change your notifications priority in your device Settings.
For the classification, navigate from the home screen of your device:
Menu --> Settings --> Sound & Notifications --> interruptions
To turn interrupt mode off quickly, you might be able to press once on the volume button and then select from
- None
- Priority
- All
Select the entry "All". Finished!
The star icon is now hidden and the interrupt mode disabled.