What's the goo on the ground cable in my breaker box?
It looks to be anti-oxidant that's gotten...crusty

Normal practice when terminating aluminum wires (such as the feeder to your condo) is to use an anti-oxidant paste or compound when making the connection -- this is said to keep aluminum oxide from forming between the joint surfaces and causing a high-resistance (read: bad) connection.

What you're seeing is likely an excess of anti-oxidant compound from the original installation, that's simply accumulated crud over the years.  You or your electrician will be cleaning it off and putting new stuff on as part of the installation process for the new panel.