Assuming that you have no control on the FTP server, one way to strengthen access security could be deploying a separate "credential server" (CS) which will return those base infos such ftp server name to contact, user and password for it.
A very good candidate to build such a CS server is B4J. You can easily add encryption to data returned to your app.
Disassembling your app would only reveal the CS address, nothing about the FTP server and its credentials.
If an eventual bad guy is going to sniff on port 21 (can you change it?), instead, that will reveal your secrets..so you should move to SFTP or other solution.