Don't over tighten the grub screw. The threaded grub screw hole in the plastic plug pulls the screw down as you turn the screw. Once it's snug against the fin tab, if you keep turning it, it has nowhere to go, so it tries to pull the plug up the threaded screw. With the entire plug anchored in the board, the material surrounding the screw takes all the stress... until it fails. When it fails, it fails at the weakest point... the thin strip between the screw hole and the tab slot.
If your local shop can replace it in a few days, get it done now. Do you want to take a chance on losing your fin during a session on a surf trip?
I agree with LB and live4truth fix it, but just as an idea since it can't hurt, why not drill a hole and thread it on the other side of the plug? I want to say it's a 5/32 tap and die set, but I could be wrong. $5 or $6 at at the Home Despot. If you really wanted to go all crazy, you could fill the old hole with JBWeld which would seep into the crack. Just tape off the inside off the slot.