One little plant definitely won't do anything, don't worry!
And bloo, my red betta has that pale white(yet slightly reddish) lining ALL around his end-tail. It's not healing nor is it infected with fin rot or anything - it's his natural lining. I find it odd, but it's been like that for about 3 months straight. Nothing changed. However, without a picture, we may be talking about different things??