Samsung S9 plus Battery

[Aaron257]

This symbol appears after I turn off my charger ...

 I usually charge the phone switched off ..

Does anyone know what the symbol means ??

[makeanimationssmooth]

just in case, clear device cache partition

[Aaron257]

I do that regularly

[carr0498]

Since you unplugged the power supply, i'm assuming the device is notifying you that it's no longer charging.  Don't think it's a cause for concern.