That's not a crack. CZ staked the bottom of the slide where the ejector pin resides to keep the pin from moving downward.
If the ejector pin ever moved down, the results would be catastrophic. The ejector retaining pin would crash into the sear cage and probably do irreparable damage. The pin should always be removed from the bottom and driven out the top of the slide. The untrained are probably not aware of this, but a gunsmith would spot this immediately. The staking looks a little rough, but will do no harm.
BTW, the breech face of the slide is heat treated about 1.0" to the rear which would include the ejector pin; this area of the slide is hard as h_ll, which also makes staking more difficult.