Red jelly like thing on rock. What's it?

[Pimba]

 

[vetteguy53081]

It’s sponge. To stop it in its tracks, expose to air and even scrub off outside of tank to reduce chance of toxins
OR
With syringe, inject air or vinegar into it

 

[WheatToast]

Personally, I like to keep sponges around (I have never found them invasive), but removal is perfectly fine.

 

[Pimba]

Why are they bad in a tank? Will fish eat them

 

[WheatToast]

Why are they bad in a tank? Will fish eat them

There are chances that they might smother corals and other sessile organisms as they grow, releasing toxins to open up space, but I have almost never heard of this becoming an issue. However, since they are filter feeders, they often starve in newer systems (but there are sponge foods available) (there are also photosynthetic sponges). Additionally, exposure to air usually proves fatal as this traps bubbles within their pores, preventing them from feeding . As for benefits of keeping them, they use silicates to build their structures, thus they compete against diatoms (brown dust algae).

Some sponge eaters I know of:
Angelfish
Butterflyfish
Moorish idol
Some starfish
Some urchins
Some nudibranchs
Sea turtles

 

[WheatToast]

Oh, could we also see a picture of the red jelly underwater?

 

[AbdominalSnowman]

Sounds like it’s time for the OP to get a sea turtle…

 

[Pimba]

It seems it never moves. I need go take the rock out to photograph him as it's hidden otherwise.