What's this!?

We put the bins out this morning and as I watched my partner put more recycling into the bin, I noticed this kind of 'fungus' on our red cherry blossom tree. Can anyone help us to identify this as we love our tree and would hate to lose it so would love to treat it correctly.
As for everything else today, my dad is nipping round to put together our new reclining chair from Dunelm, I have 1500 word assignment for my MA to do and my partner will be making a dent in her millions of work e mails. What are you up to today? Whatever it it, have a wonderful day!