I know exactly what that is.
One of your sliders had failed and that part appears to be from a stereo slider (so 1 of the 2 channels is probably still working). While it would not affect your FM stereo reception per se, depending upon which slider it is, you will probably notice that either the bass or treble on one channel is non responsive (check them separately) or it could be a different slider since those Sharps have so many.
But in answer to your question -- that is an internal slider feeler that has fallen off of an Alps slide potentiometer. A common problem with those cheapy Alps controls. To fix it, you will have to remove and rebuild your slider.
As for your FM not capturing stereo -- your 19khz pilot signal adjustment has probably drifted from original setting and is no longer centered at 19khz.
Try this to correct that: