Dear Debashish,

Eutima sapinhoa Narchi & Hebling, 1975 is a species that is found usually attached to shells of bivalve, or even for that matter on Gastropods..

http://link.springer.com/article/10.1007%2FBF00393755#page-1

another genus - Eugymanthea

Species identification of bivalve‐inhabiting marine hydrozoans of the genus Eugymnanthea - 

https://www.researchgate.net/publication/229980516_Species_identification_of_bivalveinhabiting_marine_hydrozoans_of_the_genus_Eugymnanthea

Best 

Deepak

Article Species identification of bivalve‐inhabiting marine hydrozoa...

Dear Debashish,

your species seems an anemone. (in attachment Eutima figures).

Best regards,

Cinzia Gravili

Looking at the base, I go with Gravilli that it looks like an anemone...