Provide more background information. What else is in tank?
Where did live rock come from?
Also, full tank shot with white light and close up of encrusting growth.
Why? Normal healthy ecosystem that suits them.
What does growth feel to the touch? Is it spongy, slippery or slightly rigid?
I like cryptic sponges for their biochemistry & biofiltration. I have several species of ornamental sponges: red tree, yellow ball and orange elephant ear.