Welcome to Just Answer, I will do my best to help with your problem
The best option to keep the shower enclosure tight is silicone caulking. There are a number of colors available that might be compatible. The other option is colored epoxy but it's much less available and much more difficult to work with and MUCH more expensive
Whatever you use be sure the crack is dry and clean or nothing will bond properly
I would fill the crack til the caulk (or epoxy) is just proud of the surface then use a plastic putty knife or bondo spreader to scrape it flush with the surface.