On my #7 cylinder the ceramic around the middle of the plug is loose. could this cause my car any trouble such as rough idling, decreased performance, or any other problem?

 

All of the above. Has you CEL came on? I cracked mine and didnt know it and ended up changing my wires for nothing. It ran good til I put a load on the engine.

 

Thats the exact spark plug i cracked after my header install.

Symptoms include:
-Barely beating your friends rustang
-Running on 7 cylinders
-Violently shaking shifter under load

It's approximately the same as having a burnt wire, except it's probably easier to replace.

What are you waiting for? slap a new plug in there...........

 

I would replace that plug ASAP... It will surely cause all of those above symptoms and then maybe more if its ran like that for a while....