What is this white fungus on my oak tree?

I think it might be Woolly Aphid, specifically Woolly Oak Aphid, especially as you're in Florida. The only thing I'm not sure of is I can't find anything that says this particular form of woolly aphid affects the woody parts of the tree; other forms of woolly aphid do so, coating branches with layers of what looks like white fluff or fur. Whilst a bit technical, this link here woolly oak aphids, lower down under Control, suggests contacting Florida University extension office to decide if it needs treating and what to do about it - with most forms of woolly aphid, spraying the branches with the hose is enough to dislodge it, but this form affects the leaves of course. They may also be able to help regarding whether or not it forms deposits on woody parts of the tree as well as the leaves.

I do have one other reservation though - the fallen leaf covered in fungus does not look like an oak leaf, unless its shape is somewhat obscured.