I have metal halide fixtures that have a clear glass. I
would like to make some sort of a light diffucing
glass and am curious if there are
any paints or coatings that stick to glass and
are white and diffise light.

You have to be very careful - even clear glass gets quite hot in such a
fixture and by changing the glass you probably have to re-do UL
qualification etc.

That said, the keyword you want for diffusion in such a fixture is
"frosted" or "prismatic". Glass remains clear but is cast (prismatic)
or sandblasted (frosted) in such a pattern that internal
reflection/refraction takes care of diffusion.

Tim .