Sponge on frag- are they bad?

[becact]

I thought I read somewhere you do not want sponges in your aquarium. I just got a frag with a purple encrusting type of sponge on the plug and growing onto the coral. It is very soft, almost turns into a liquid when you touch it. Should I scrub it off or just let it be?

[400M1963]

Sponges are livestock you can purchase. Once exposed to air they can be harmed. Too many harmed sponges can mean the die-off lengthens LR curing. Once cured the LR can sprout sponges that were not visible or the ones that are there will grow.

[becact]

I guess my real question is, will it overgrow the coral? It is a green birdsnest.

[400M1963]

These encrusting sponges are welcome but don't grow big and fast. It's my experience they tend to like shaded places under rock. I treat them like bonus bio-diversity in tank when they pop up - another indicator of health of rig.