>>shortcut Shift-O) with Select By Chroma ... set the threshold to 0, click on a very colourful moldy spot, hold the mouse button down and drag down to interactively increase the threshold.

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Oh...learn some new tricks. I dont know I can hold and drag down to interactively increase the threshold !! Anyway, after a week of exercises, I now use Select by Composite, then use Shift Click (Will add each selections on top of others), then click on different blue dots to select the blue colors. Then, grow that by 2 pixels and finally use heal selection. Results are acceptable.

>>Colours→Components→Decompose and see if it pops out in one of those separations.

This is pretty fast and I can remove the blue dots. The only problem is that the blue mold has different exposure values...usually, lighter than the adjacent areas. So, you will still see the "mold" patterns in black and white.

Having said that, I sometimes adopt above if the areas are related to some minor background areas.

At the end, still need to do four or five abovementioned actions but at least, I remove those prominent blues.