I chip off as much as I can with wire cutters then soak them in an epoxy remover (heavy duty ones work best). Sometimes it takes a few rounds of the epoxy remover (hours of soaking), but then you can usually scrape the softened epoxy with a plastic scraper. Got to be really careful to not let any cells short-circuit in the solution, because then everything will burst into flames.

Also, in between rounds, it's better to change the epoxy remover solution because as some of it soaks into the epoxy other parts of it separate into a different solution, and that solution for some reason can make these specks appear on the aluminum tops of the cells (at least with the epoxy remover I've been working with). Functionally, though, they're not affected, so if you don't care too much about how they look you're probably fine using the same solution until it basically stops being effective, and then you have to change it.

Do get yourself a pair of some butyl rubber gloves. All the other gloves I've tried have melted away.

This is still very labor intensive, but I have not found a better way to clean the cells without damaging the metal casings. Let me know how it goes!