It would remain and the effects of Leech Seed would continue until removed by another mean.
This fact has been confirmed by various users who have tested this.
>Forest's Curse cancelling Leech Seed didn't even cross my mind.
Just levelled a Phantump to test it and can confirm that leech seed REMAINS on the opponent :)
Benzychenz, Nov 11, 2013
>Yeah, Forest's Curse won't cancel Leech Seed's effects, like how a Poison-type remains Poisoned after switching out of Soak.
Punchshroom, Nov 11, 2013
Smogon Forums o3o
Here's a quick replay of it in action