Often times they'll get glitched and be stuck somewhere, never moving from the spot. Not sure why. Sounds like this is the case here.
Normally though, they'll either trade ships with you for minerals and energy or ask if some of their pops can settle on one of your worlds (or 'orbs' as they call them), sometimes they'll ask to settle on a brand new planet I think.
Also, a warning, there is a bug with part of the trading where there is an encrypted commuque (not sure what triggers it) where they ask for cash in exchange for more ships, however, it fails somehow and you get ripped off when it gets to that extension. Every. SIngle. Time. The devs have yet to fix it for some reason. *grumble* *grumble*
Since it sounds like they're bugged for you, you can kill the fleet if you want.