Usually yes, but not because it’s an L lens by itself. Most current Canon autofocus L lenses use ring-type USM, which supports full-time manual focus, so you can turn the focus ring even when the lens switch is set to AF. TS-E L lenses are manual-focus only, so this doesn’t apply to them.

A simple rule: if the focus ring turns freely in AF mode and does not fight the motor, it’s generally safe. If the focus ring moves during autofocus, or if it hits hard stops at minimum focus distance or infinity, only adjust focus when the lens is switched to MF.

So the key is the lens design, not just the red ring. Ring-USM and lenses specifically marked for full-time manual focus are safe to tweak in AF mode. If you have to force the ring, don’t do it.

Also, if you manually tweak focus while the shutter button is half-pressed, that focus can be maintained for the shot; if you release and refocus, autofocus may override your manual adjustment.