What is yellow thing and how do I remove it?

[Shon]

Sooooooo many species of sponges...

he said it was inside the skeleton

https://naturalhistory2.si.edu/smsfp/irlspec/Cliona_spp.htm
"Boring (burrowing) sponges create a canal system by chemically etching out coral or shell material for shelter.."

" capable of encrusting the surface and smothering the host."

 

[TvanB1]

he said it was inside the skeleton

A healthy coral wouldn’t allow anything to grow inside of it lol. It was either dead or dying beforehand.