Help?

[stoof_helicon]

How can I avoid/fix this "fog"? Tips?

Thank you! (censored filenames for work purposes)

[Catherine]

This is due to focus breathing of the foreground part occluding the background part. The first thing to do is check whether there even exists an image in the stack where that part is sharp. Often there won't be one, which means it's not a problem with the focus stacking algorithm and has to be fixed during shooting (for example, by shooting with smaller aperture or at lower magnification). But if there a source image that contains the missing details, you can use retouching to fill in these parts.

I also see that there are warnings for for some of your stacks, fixing those may help, especially if the warning is about the images being out of order. Hover your mouse cursor over the result with a warning sign to see the detailed warning message.

[shutterbug]

I would try using rendering Method C (pyramid) instead of B.
That's better suited for a subject like this one where you have a lot of overlapping complex structures (twigs)