If you cannot take out the driftwood:
Figure out how much Excel OR hydrogen peroxide is the maximum dose for your size tank.
Turn off filters, power heads and any other water moving equipment.
Use a syringe without the needle and squirt that much Excel OR hydrogen peroxide right into the densest parts of the BBA. If there is still a lot of BBA you might not get it all treated in one go.
Let it sit for a while, then turn the filters etc. back on.
The next day do a partial water change (25% minimum) and repeat. If you were not able to treat all the algae the first day, you could treat the remainder today.
When it turns pink, it is dead. Sometimes it takes 2 treatments like this to kill it. Sometimes it dies with just one treatment.
If you have so much BBA that it will take more daily treatments, then I would do larger water changes, practically drain the tank, then squirt or spray the Excel or H2O2 directly on the exposed BBA. The stronger concentration of either of these materials when the algae is in the air can kill it better that gambling that the water is still enough to maintain the concentration when you treat a full tank. When you do it this way (drain the tank, treat exposed algae) you can use about double the amount of H2O2. Then wait until the bubbling quits before refilling. Most of the H2O2 will have changed to water and oxygen so is not toxic.