A vacuum and a paint brush do a nice job on this. Vacuum first without touching the fins, then use the brush to clean between the fins, catching the fallout with the vacuum. The bristles of the brush reach deep between the fins without bending them.
If necessary, after you get what you can dry, you can use plain water and see if you can get a little more. ...
What worked for me were Q-tips. I worked them around the surfaces of each fin and attempted to do the same with the pipe to which the fins are attached. The shaft of the Q-tips were long enough to reach beyond the top to bottom of the fins, and the tips were wide enough to touch both surfaces of adjacent fins without damaging them. The process was not fast ...
From experience, from inside the fridge you can clean it out with a cable tie (flexible long thin piece of plastic).
Insert tie and rotate.
It is unlikely to be blocked all the way down, just at the join at the top.
These appear to be rust stains. I've had reasonable success with a product called Wink:
Wink on Amazon
there are also similar products that may be as effective but I have not tried them personally. I do know that chlorine bleach does not work on these.
Be VERY careful with this product, however. It contains HFl (hydrofluoric acid) which is nastier than ...
I would use a razor or scraper but with great care.
You can do just enough to get the residue off but it might be easy to damage the surface treatment.
Some of those are actually enamelled and very tough.
As commented, this is usually part of the mitigation company's deal, as the tools are a bit specialized (serious ozone generators, for one thing) and they may have access to ones a bit more serious than any you may be able to find to rent, as the rental company may not be comfortable with the hazard level of putting those in consumer hands.
Other methods ...
That framing will last decades with no maintenance. I just replaced the decking on my mother's 20-year-old pool deck which had not been well-maintained through the years. The framing was all in good shape though the decking was rotted and warped.
With that said, if you want to wash it for cosmetic reasons, it won't hurt to do that.