Does a single photon slow down in glass ?

https://youtu.be/CUjt36SD3h8

Inside the glass, no

It does have a lower speed in glass than it has in air or vacuum though

No. The explanation often used is that internal to the glass, photons move at the speed of light, they are absorbed and briefly held by electrons of glass molecules and then emitted again, and this is repeated by successive molecules, which thus slows down the journey through the glass. And while this is quite a satisfying and believable explanation, as I understand it, the real explanation is just a quantum mess whereby the photon enters the glass and later another photon is emitted the other side and it is all the mathematics of the wavefunction of the glass.

Yes, a photon slows in every medium.

The speed of light is 3*10^8m/s in a vacuum and slower everywhere else.