Cleansing Foam is the third upgrade on the first path for the Monkey Engineer. It allows the Monkey Engineer to shoot a blob of foam that can remove Regen and Camo properties of bloons, as well as pop Lead Bloons. Only 3 blobs of foam can be on the track at once (unless the Monkey Engineer has an attack speed boost). If a certain amount of bloons go over the foam, it disappears, regardless of if they are Camo, Regen, or Lead.
In the Flash version of BMC, the Engineer's Foam has no limit of amount of bloons passing by. This makes the upgrade super powerful, effectively removing all bloon effects if placed at the start of the track.