Lasers are beams of highly collimated, or 'coherent' light.

In real life they're only visible because (a) laser shows generally happen in dark surroundings and (b) the atmosphere scatters some of the light off its path towards the viewer. They normally turn on the smoke machines so the lasers have plenty of tiny particles to get scattered by.

In normal conditions - room light or daylight - in the scenario you describe, you'd see a faint spot on the mirror (scattering by the glass) and another spot on whatever surface the laser is reflected onto; the  beam itself would be somehwere between hard to see and invisible.

All that presumes that it's a visible light laser. A high-energy laser weapon would probably work in the far infra-red, and therefore be invisible to you and me. Whether such a beam would destroy a mirror or not would depend on how reflective the mirror was (or how little of the energy in the beam it absorbed).

The laser weapons you see depicted in Star Wars etc. would almost certainly not look like that if something similar could be developed for real. To get that effect in Poser, you'd need to make a prop (an elongated cylinder) and a shader setup to get that glowing effect.

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