If you reboot the phone does the problem go away immediately?
You may want to turn the phone completely off, remove the battery, and then use a needle to carefully clean out the headphone jack. If you have a tiny piece of cloth you could dampen it (don't soak it) with isopropyl alcohol and run it in and out of the connector a couple of times to clean any residue off of the contacts.
If none of that helps then I'd suggest a Factory Reset (particularly if rebooting the phone makes the icon go away).
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