mussel

What is this. looks like a mussel or clam. not sure how it got in tank either. wasnt sure if its harmful to anything. 120lit 0 amm 0 nitrites 5-10nitrates kh& gh is fine ph 7.4 stock is 7 corydoras, 1 bristlenose, 7 rummynose, 3 zebra nerites, 6 cherry shrimp