If I catch what you mean, then you'll want to use an Adjustment layer with the mask on it. Put the adjustment layer above the bubble, and have the bubble Alpha Inv. Matte the Adjustment layer (or just Alpha Matte, depending on how you made your mask).
If I'm posting a question, it means I've Googled it for far too long and couldn't find a solution/answer.
Precompose all of the bubble layers first (select them all and press Ctrl+Shift+C). If that background layer has transparency, duplicate it and use it as an alpha track matte for the bubbles layer. You'll need to reveal the “Modes” column to use track matte. In the timeline above the layers in the area that says layer name, parent, etc, right-click > Columns > Modes. If the background doesn't have transparency, then just trace the mask onto the precomposed bubble layer.