Close a hole in a matte

The sequence of choking and spreading occurs in two stages, each with its own set of identical controls. Typically, stage two does the opposite of stage one. After a specified number of back-and-forth adjustments (which Matte Choker handles automatically), the hole is filled and the matte shape is preserved.

Select the layer, and choose Effect > Matte > Matte Choker.
Set stage-one controls (the first three properties) to spread the matte as far as possible without altering its shape, as follows:
Geometric Softness

Specifies (in pixels) the largest spread or choke.
Choke

Sets the amount of choke. Negative values spread the matte; positive values choke it.
Gray Level Softness

Specifies how soft to make the edges of the matte. At 0%, the matte edges contain only fully opaque and fully transparent values. At 100%, the matte edges have a full range of gray values but may appear blurred.
Set stage-two controls (the next three properties) to choke the matte by the same amount as the amount by which you spread it in stage one.
(Optional) Use the Iterations property to specify how many times After Effects repeats the spread-and-choke sequence. You may need to try a few different settings so that the sequence is repeated as many times as necessary to close any unwanted holes.