"So-- insulation against the pipe-- okay or not okay? "

You stated that the pipe is already against a concrete block exterior
wall. If you put the insulation against the pipe and cover up the pipe
and block, now you've insulated the pipe from the warmer air in the
garage and made the problem worse. The insulation will keep the cold
outside temps confined to the block and the pipe, so it will freeze
even faster. Fiberglass insulation against a hot water pipe is fine,
it's done all the time with pipes that run through exterior walls.

To be effective, you need to get the insulation between the pipe and
the block as a first objective. If you can get access to the pipe, I'd
see if you can move it enough to be able to get the tube type pipe
insulation around it. If not, can you cut it and move it out 3/8 of an
inch? Or else, reroute it, or use electric pipe heating tape as
someone suggested.