The best thing would be to remove the motor from your motor, and then take the bell of the coils so that you can properly clean everything. Using a toothbrush will be fine but you can also use some compressed air to try clean the dirt.
Once you have cleaned it you might want to add some oil to the shaft and bearings.
I am not sure about the motors on the runner 250 as I have not tried to open them myself, but most motors are very similar and have a C-clip on the bottom that you need to take off. You will need to remove the motor from your frame.
So, I could manage to unscrew the 4 screws underneath and dislodge the motor from the platform, but I don’t know how to unmount the grid ontop (the part with a yellow sticker on your photo) of the coils to clean the dirt thoroughly… There is something like a small handle you could lift it underneath but it wouldn’t budge with my nails and I didn’t wanna force it. Do I need that tool shown near the precision screwdriver to unmount the grid from the motor?
about ½ a mm from the small hole where the shaft goes through , yo will see the bit that spins thats the bearing , make sure it turns smooth , if not there may be GRIT IN THERE ,
merry christmas happy new year and happy flying