In the update 7.2.0 the following robots got an extra slot for passive modules, so they now have 3 passive module slots instead of 2 as it was before:
- Ao Guang
The new extra slot is not equipped with any module, and that's the reason you're seeing an icon of the unused slot:
Please open the modules tab and check the lowest module slot: it's probably what's causing the icon to appear.
In order to make an icon disappear, you need to install a passive module into an empty slot.
However, if the slot is locked due to the level of the robot being too low (as in the screenshot below), you'll be able to equip it later, when your robot reaches the required level. Until then, it won't be possible to equip it.