Calcium carbonate scale is rarely white. It can be many colors depending on what impurities it picks up. Certain forms of copper oxide can appear dark brown or black and so the scale may have picked up some metallic impurities.
You should be very careful about descaling the heat exchanger. Unless you verify that there is calcium build up by pulling off the manifold and inspecting the copper tubes, then there’s no reason to descale. All heat exchangers will build up a thin layer of scale on the internal surface that is protective by nature. Stripping that scale down to the bare copper metal will only do more harm than good.
I would suggest you simply flush the heat exchanger with clean water to remove any loose particulates and then make sure the manifold is clean. Then put it back together and leave it alone. Your water shouldn’t be scaling out calcium at the levels you have posted but if you want to reduce CH, then drain the spa and refill it with fresh water. If you do that a couple of times then the entire pool CH will start to come down.
