Press Alt F2 and type in "free the fish"

[lah7]

Or just reload the panel by pressing ALT+F2:

mate-panel --replace

There's also this old topic:

The Hidden Fish

There’s a little fish hiding on your system – making its journey throughout GNOME 2 to MATE. To let the fish swim for a while, press ALT+F2 and type: free the fish Wait a moment… oh, hello, there you are. Pretty cool, huh?