This seems to be a filter capacitor and that explains why the card works even though thats been knocked off.

Is it a major issue ? well that depends, given that it is on the right hand side of the PCIE interface, that means it is used for filtering signals and not the power supply. So what could go wrong? it could be nothing or may be something as simple as missed frame or something as bad as a BSOD.

Remember that manufacturers have done everything possible to reduce the number of components on the board to reduce cost. So each component on that board serves a purpose in meeting the specifications advertised for that card.

Basically here is how i would have handled it (assuming that I do not have the skills to repair it on my own)

- If i was not using the card to its full capacity i,e use it playing games at FHD and not 4K and lower refresh rates eg: 60Hz. I would let it go and continue using the card as it is

- If you are using the card to its capacity then If you have access to a repair technician with the skills and reputation like Alex/Northridgefix, take it/mail it to them and get the capacitor back in place

- If you dont have access to such a skilled repair technician then live with the issues that may or may not come. because a person who is not very skilled with graphics cards could do more damage than good trying to replace the cap. 

Hope this helped.