Probably a great idea to have the tail rotor g/box inspected thoroughly before any more flight if debris found after cleaning plug. I'm not an a/c engineer but I would imagine it's possibly a pre-warning of something more serious. Usually, bits of 'fuzz' find their way onto the plugs in g/boxes etc, but after cleaning the plug, none should be found so soon after. That would apply to all heli types, not just h500!!
Check if second lot of particles are ferrous with a magnet. Again, not sure what components are made from but my advice? GET IT CHECKED before flying it again.
