I would consider this a bug so as far as I know the support forum is the best place for it, my apologies if it belongs anywhere else though. However, I doubt it’s that high priority of a bug as it effects only a corner case of GSB modding.
If you make a mod that makes it possible for a ship to have 0 credit cost (it’s not too often you’d want to do this, but I’m sure the occasional use could come up), when you build the ship within a Galactic Conquest campaign, the ship’s fleet icon glitches.
If this is the very first fleet you build, the icon has some random white bits in it, looks different when clicked on than not, and in planetary view, looks like one of the buildings. If the costless ship is built AFTER you have made another normal ship (in a separate fleet), then the costless ship’s fleet icon becomes whatever the icon of the last built cost-having ship was. If the costless ship is split off from another fleet, it copies that other fleet’s current icon. (And I think pressing the turn key resets the icon of a fleet containing only the costless ship to the aforementioned weird icon with random white flecks in it, but I didn’t test that too much.)
This also occurs if you manually edit the text files for a campaign save to include the costless ship within a campaign save, so it’s not the construction of the costless ship, but the mere existence of it, that somehow messes up its Galactic Conquest fleet icons.