What are the security implication of having the port 523 reachable from any address in my network?

Is there a way apply a list of allowed ips and block the rest?

Are there other ports I should consider to close?

That depends on what listens on port 523 and what it does when it hears something.

Certainly. Firewalls are the specifically for that purpose.

Normally you should close all ports, except those that you actually use.

P.S. What does it have to do with DB2?

523 is the port for administrative instance;