How do I remove this shower valve stem?

I attempted to disassemble my hot/cold water faucet valves in order to repair a leak. However, I got only so far to a point where there appears to be no other nut to remove the stem! (See picture). The stem itself is round with no flat edge to attach a wrench.

Is there any way I can proceed? Or do you advise I call a plumber at this point?

can you show us the other part you took off? from 2 or more angles.

Take a wire brush and clean of the some of the crud.
are there any flat edges on the outer round part.

You may have to take a flat tip screw driver and chip a little tile off to make the hole bigger. This is where it gets tricky. Put the blade of the driver as close to the hole and whack it with a hammer. chip off little pieces at a time to make the hole around the stem bigger. You try to take to much off at a time you might pop out a whole piece of tile.

On the escutcheon you took off, Is there a fitting or a threaded part that maybe still attached to the underside of the escutcheon?

The round sleeve unscrews. The nut to remove the stem is behind it. You will need a deep socket to get at the nut. I think it's a Sayco faucet.