So, if I'm getting this right, I don't play the FF mmo, being grey means you are so bad a f2p player is better than you despite you paying the monthly? or did I get that wrong?
Its a third-party tool, not an in-game measurement. The devs are aware of it, and they know they can't reasonably ban people from using it (They outright said in a livestream that it'd be like banning people from using calculators- all it's doing is adding up everyone's DPS numbers, just a lot faster than doing it by hand) but they highly discourage people from using it to be jerks to other players (i.e the "you parsed grey, gtfo" kind of talk). If you want to use it to see if you're doing your best/to see how good everyone is in general, they're fine with that; if you want to use it to be a jerk to other players who aren't in Super Tryhard Mode, they are not fine with that.
Grey means you performed within the bottom 25% of players to ever do the content. So you have to be bad at the game but also you still cleared it so only elitists really care.