What do you mean with that? Where is this "bronze dirt" on the deco plate?
I mean: the deco plates on both ZF (blue&grey) and GF (black) are not made of stainless steel. If the watch is not airtight, the deco plate would get rusted. It can be sooner or later to get some rust stains on it or inside it, and then the particles resulted from the rust can fall into the movement, obstructing its function. Your watch would stop working then.
GF allegedly sorted out this issue by using a refined deco plate.
Who knows. It may be but a marketing ploy by GF to outmatch ZF, the QC from ZF has indeed been disastrous for a long time though, especially with their IWC reps.
Using Hangzhou 7xxx is truly good news. The movement is a Chinese in-house, cheap, robust, and new. It has modern specs with a long PR. Its instant date-flip and bi-directional winding make it a lot better than the freaking noisy miyotas with the incorrect progressive date change function.
I hope they replace everything made with miyota with HZ stuff or 2892 (ST18 series) in future.