The amount of calcium and magnesium in Seachem Flourish are in very low trace amounts, usually not enough to accumulate such that it increases GH noticeably (unless you are massively overdosing it everyday). Any minerals in the ferts should be used up easily by the plants. I've used Flourish in my planted tanks and in some setups dose 3-4x more than recommended dosage due to higher densities of plants, yet the GH levels didn't increase.
Based on the photo of your tank, the GH increase is most likely due to those rocks. Those types of rocks are commonly known to release minerals over time. You can even see the white calcareous veins on the rocks themselves.
A higher GH level is usually okay for most hardy livestock as they can adapt to it if the changes are gradual. But it could sometimes be an issue for certain sensitive livestock like shrimps which may encounter molting difficulties at higher GH levels.
RCS are generally hardy and adaptable so they could adapt to the higher GH levels, but if you find that they are starting to die off due to molting issues, then you'll have to gradually reduce GH with water changes and consider removing or replacing those rocks.
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