Clam growing on my Clam

[IronChefItaly]

Do clams reproduce asexually or would you need to have more than one squamosa clam? Could otherwise be a standard hitchhiker. Maybe try to count the ribs and see if its the same number.

[NU-2reef]

Looks like an oyster. Should be easy enough to deal with but I don't think that's why the clam hasn't been opening up much recently