Ya, that looks like the silicon is steadily separating, and is pretty serious. Luckily it has given you ample warning but looks like that seam will suffer a catastrophic failure in the very near future. When it happens the whole side will open up like a zipper and dump the whole tank on the floor.
It is possible to fix it, but not with stuff in it. To fix you would need to take the tank down and disassemble the glass panels, remove 100% of all the old silicon, and then reassemble.
So comes down to whether you want to go through that hassle, or just get a new tank. Either way it looks like you might want to decide fairly soon. If the patch is growing in size, then you have a limited short window to come up with a solution before it dumps everything on the floor.
You can alleviate the pressure on the seam a bit by lowering the water line as much as possible. That will buy you a bit more time, but still needs to be replaced asap.
