You mean the one on the left looks cleaner? I was thinking that the middle one looks fouled, but agree with Boobie -- hard to see a lot in the pic. If one is super clean, it could be washed/steamed (water intrusion). And maybe it's not holding up its end of the job -- not firing.
Conversely, if the middle one is fouled, maybe that one is periodically not firing -- getting a buildup of unburned fuel, some carboning or goo-on-the-way-to-carboning. Maybe that plug simply wasn't torqued down as well as the others, or was gapped wrong.
I would check compression and spark using appropriate testers, all plugs removed -- open air gap tester for spark (cheap little device). If the ignition is producing strong spark all around, replace the plugs, gap to spec and try again, assuming the compression is uniform. Post your numbers.
(I don't see any reason why those plugs would be unsafe for the motor somehow, so not sure what you mean there. Can undoubtedly use cleaning and regapping if reusing, but that's just standard stuff.)
