As @Adrian mentioned, you need to set FileZilla to use an encrypted connection when talking to this FTP server.
Quoting from the FileZilla wiki:
For a client to connect to a server using SSL, then the host for that connection needs to be set to FTPS. In FileZilla client this means prefixing the host with "FTPES://" for "explicit" FTPS, or "FTPS://" for the legacy "implicit" FTPS.
FTPS (SSL/TLS) is served up in two incompatible modes. If using explicit FTPS, the client connects to the normal FTP port and explicitly switches into secure (SSL/TLS) mode with "AUTH TLS", whereas implicit FTPS is an older style service that assumes SSL/TLS mode right from the start of the connection (and normally listens on TCP port 990, rather than 21). In a FileZilla client this means prefixing the host with "FTPES://" to connect an "explicit" FTPS server, or "FTPS://" for the legacy "implicit" server (for which you will likely also need to set the port to 990).
Depending on how the server was configured, one of these two settings should work:
ftpes:// on port 21
ftps:// on port 990