It's likely that the enemy culture hasn't been fully neutralized. Check the phase lanes colours; if the line connecting to your planet was the enemy's colour, then it's still under enemy culture. I'd expect that your culture lanes haven't reached the planet, while the enemy culture was only starting to be neutralized.

Hostile culture will retreat, but that doesn't mean the planet they originated from will immediately be relieved of its influence.

Dead end planet spread their culture most effectively.  It is all going in one direction.  It isn't split in halfs, thirds, etc as it spreads to adjoining planets, depending upon the number of phase lanes there.

Here's a strategy I've used on a dead end planet for crushing culture spread.  Sneak an Advent Star Base onto to a dead end planet behind their Home World with the Culture Ability upgrade.

It looks like the Media Centers on your homeworld haven't been built yet.  Altho they are qued up to be build, the constructor ship hasn't completed its other tasks yet (also a whole bunch of unbuilt turrets too), or gotten there yet.

Does the opponent have Media Centers on that green dead end plantet (a terran)?  Altho you may have liberated that planet, I believe his Media Centers will continue to broadcast culture despite the planet being dead. 

It's a slim possibility, but if you lost your homeworld, then you'd be losing culture at that rate. Check to make sure you have a homeworld on that map. Probably isn't the problem, but worth a look.\

Neeeevermind. Just checked the image. You have one.