I was having the same issue, but I was able to make an exception to open ClanGen anyway. I googled around and found this:
To change these preferences on your Mac, choose Apple menu
> System Preferences, click Security & Privacy 
, then click General.
In the General tab at the bottom it said something like "open clangen" and I clicked it and then reopened clangen and was able to actually open it. sorry if I explained that poorly haha