When SyncNow's server cannot connect to the database directly, which is common on shared hosting where MySQL accepts connections only from the site itself, it goes through a small gateway file it places in the project's folder on your hosting over FTP, named syncnow-db- followed by random characters.
This is why a database often needs a working FTP connection too: without it the gateway cannot be installed and the database cannot be verified. The gateway accepts calls only from SyncNow's server and stores no password, and a refusal seen only from the SyncNow server, without going through the gateway, is not treated as proof and does not light the yellow sign.
Fix it in the project's Connections page: in Databases, click Edit on the database, correct host, user or password, click Test connection, then Save; Add database adds a new one. A connection is tested before it is saved: if MySQL rejects the user or password it is not saved and the window says why, and if it cannot be verified at all you can still choose Save anyway.