No not bba.
It’s a bacterial / fungal bloom from the new wood. You can scrub it off and, or shrimp will eat it.
That is good news!
I’ve no. Experience, but amano maybe. Rainbows have small mouths so may be ok.
Nerite Snails (not pest ones) are good for cleaning wood and rocks.
Nerite snails are ok. You get loads of different sizes and colours. Some with horns on them.
They need salt water to breed so no you won’t be be over run!
Nerites will give you hard to remove white eggs on the wood.