How to connect to my web app running on mac over WiFi?
There could be a few reasons, but your security settings are probably the most likely candidates.

Temporarily turn Off the Firewall

The easiest way to test this is to (very) temporarily turn off your firewall:

System Preferences > Security &amp; Privacy > Firewall > Turn Off Firewall








Retest your connectivity, if it works, turn your firewall back on and then allow the app through the firewall.