To answer your last question first, yes, it would be best to start with green energy first. However, 1) that isn't an option for everyone at this moment in time, and 2) if/once we achieve a green-energy based electricity grid, this exchange would still allow those with idle processing power and excess energy production to put their assets to better use.

To address your main point: you are correct that such an exchange alone would not affect the power creation and consumption balance. The reason for this is that in some reasonably small slice of time, global energy production is approximately static, and the percentage of each type of energy technology contribution to the total production is approximately constant. Buying/selling SLC does not itself increase the number of solar panels producing energy, hence the amount of non-green energy production would remain unchanged. However, the exchange would not be operating in a static environment.

Suppose person A pays for the electricity consumed by their hardware in fiat, and that electricity comes from a mix of dirty (D) and green (G) energy, and A's total energy consumption T = D + G. Also, suppose person B with rooftop solar is receiving money in fiat for whatever surplus energy they send to the grid (1). If A and B are geographically isolated in terms of the electric grid, it is impossible for B to actually "supply" A with the electricity A requires. However, suppose that they are in close enough proximity that B is able to supply A with T energy (2), and that global energy production remains constant. In this scenario, A's energy consumption is theoretically disjoint from the non-green energy sources (not in practice, of course). This is critical to emphasize: the energy that A is consuming is not coming from dirty energy sources. If all other energy sources and sinks were to disappear but the infrastructure to remain intact, this would be a simple exchange of energy between A and B. Why this abstraction? Well, at some geographical critical density of Gridcoin crunchers and solar energy producers, this is actually what happens, just with more agents.

But why is this important? Isn't the same amount of emissions being created? Well, if the process I just described occurs, then many Gridcoin crunchers would be in a situation where their energy is effectively coming from green sources. But if the global power creation and consumption remains constant, and the demand for green energy remains constant outside of Gridcoin crunchers, then green energy sources will have to replace non-green energy sources in order to meet that demand. This is a consequence of that fact that Gridcoin crunchers were themselves demanding that their energy come from green sources (whereas before it was coming from a mix), by trading their GRC for SLC. The demand mechanism is not occurring via fiat, but rather by trading GRC for SLC, thereby giving the latter value (or increasing its value). Ideally this would increase the incentive for installing solar panels, but in the short term it is more likely that already existing solar producers would start claiming their SLC.

1. This isn't exactly how Solarcoin works, which is why I only used it as an example
2. I am making many simplifications regarding how the grid works

The above was my original response and thought process, but that reality is, it is a shaky logic, and personally I am not even convinced by it. Maybe some modification of my proposal could achieve what I am describing, but as it stands I am not sure it holds up to scrutiny. I would have to develop the proposal more and then maybe come back with some updates.

However, after thinking about it, I realized an easier approach: Gridcoin crunchers trade their GRC for a cryptocurrency that invests in/builds/installs sources of green energy. One that I am familiar with is Solar DAO, but there could be other ones which are better suited to the task. In this case, new sources of green energy are actually being brought online, which solves the original problem I described at the beginning of this response. This actually would decrease the amount of non-green energy being produced.

There is also another way to offset carbon emissions that I was planning to address in another post eventually: crunchers sell their GRC and used the proceeds to buy/donate land in the Amazon rainforest that would otherwise be cut down, or for responsible reforestation projects (similar to how Flixbus offers its riders the possibility to offset their carbon emissions), whether through traditional means, or some cryptocurrency. This is a clear instance where GRC is being used to offset carbon emissions. If this was done with a cryptocurrency, then it would reinforce and expand the multi-currency economy. (Also, I see that @gregan just mentioned something similar in a comment below).