Let's imagine the moon was radioactive and all of its constituent particles underwent radioactive decay at the same time, with all photons exiting in directions perpendicular to the earth-moon line, OR imagine the moon is smacked out of orbit by a blob of osmium e.g. twice the size of the moon traveling at e.g. 0.9 c.
In either case, the mass of the moon twinks out of the picture quickly (conceivably its impossible for this to occur instantaneously) . What do observers on the earth notice, if anything ?