The Toxic Bloon is an Advanced Blooncard in Bloons Card Storm. If it is popped by a Monkey, that Monkey is removed from play. It does not affect Heroes or any other forms of dealing damage.
This article is in need of research. You can help out by editing this article or discussing your findings on the talk page. The editor who added this notice elaborates: Interactions with Burny Stuff Mortar's On Fire popping the bloon
Heroes and Powers aren't affected by Toxic Bloon. Quincy and Obyn Greenfoot are reliable ways to do the final pop without Toxic Bloon's tower-removal penalty.
If there are no attacks available besides monkeys, use the Monkeys to deal enough damage such that the Toxic Bloon does not get popped by one of them before it leaks, unless you are going to replace the monkey you are dealing the final pop with anyway. Note that defenders will attack a leaking Toxic Bloon if they have remaining ammo, so either use up all of their ammo on other Bloons or use Whoops to protect them.