I have had this beauty for 3 years. There has always been some sponge growth under the mantle, but recently i have begun to wonder if there is a chance of the sponge out competing the clam for real estate.
The sponge should not be competing for real estate with the clam. If anything the sponge will be restricted by the real estate (shell area) provided by the clam. As long as the mantle of the clam is not affected by the sponge (should not be) there should be no problems.