I tried to find out how to remove the mangrove, but the funny thing is that the only thing on the internet is how good they are for water chemistry.
If its not too much trouble, you may want to move that rock to where the mangrove is in the back of the tank. From a brief research stint, I have found that the root network of the mangrove is unparalleld in its ability to absorb ammonia, nitrites and nitrates.
The problem with removing the mangrove is that it utilizes root networks and runners. Miss just one small fragment of root and you are likely to have another mangrove appearing shortly. Maybe you can pull as much of it out as you can, then plug up the hole with a small rock or something.