Hi,

This looks like a green sweat bee, which is not creating the rot but taking advantage of the decay that is already there to make nests. Some of these bees, which are beneficial pollinators, nest in rotting wood.

See this page:

https://nativebeeology.com/mind-your-bees-and-gardens-2/sweat-bee/

https://www.sharpeatmanguides.com/sweat-bees

A tree that is properly pruned shouldn't have that kind of rot. Leaving the stump of a branch means it won't heal, which is what has happened here. See "how much to remove" on this page:

https://extension.unh.edu/resource/basics-pruning-trees-and-shrubs-fact-sheet

If you want an expert to try to correct his, contact a certified arborist. You can find one here:

https://www.treesaregood.org/findanarborist

Hope this helps. Good luck!