Leave the roots until they are just too big to be of use. Any plant with roots growing is alive, so leave them all be. Try misting the top, that should help it a bit. Oh by the way what is your magnesium like?
i find the better your reef looks, the worst mangroves will be. i think it is cause of the lack of nutrients in a heathy reef. careful the roots does not grow into the seams of the silicone of the sump. it can cause leaks.
if the roots becomes black, it is wise to trim off as it has rotted n will polute the water. if the stem is partly blackened, chances r the whole stem will slowly turn black n rot n it will smell.
I think those trees are going to make it. He is right though, if the roots become to large, you should trim them at that point. I think it will be a long while before you have to worry about it busting out your aquarium.