بسم الله الرحمن الرحيم
By ‘mussels’ if you mean the shell insect which is known as ghongha in Hindi then it is not halal to eat it according to Hanafi Fiqh as only fish is halal among the sea animals, whereas the seashell is an insect.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.