It's a bracket fungus, sometimes known as a shelf fungus - it spells death for the plant it's growing out of.

Sadly, no. That is only the fruiting body, the main part of the fungus is already well entrenched in the tree. It has to be to have the strength to produce the part you can see.

Agree with Owdboggy - sorry!

It is probably one called Laetiporos sulphureus aka Chicken Mushroom. Young ones growing on Oak or Chestnut are edible, but don't take my word for it.

and it is nature recycling dead/dying old trees. Sad for the tree but all part of the circle of life [ strains of The lion kng wafting over my work place] Hakuna matata!!