I guess another clam SP is attached to your one but can't confirm what exactly. Have a look on this (the selected one) how many are attached to it @KJ if isn't predator the attached one do you think can hurt his clam ?
(how I take off something like this attached to clam or another shel I want to save, I'm taking very fine/thin needle (gauge sizes 24-26) with 1-1.5ml syringe -> filling the syringe with white vinegar -> waiting the other shell to open and I stick and empty the vinegar in it. In hour max it will be completely dead and I open it gently with knife or something under the water and empty it not to pollute the tank water.
Done it few times for customers and friends.