Whats this growing under my monti?

[Willbiker]

Hi all. Could someone identify this ugly growth which is preventing my monti from growing? How can I get rid of it? Is it toxic if I disturb it?

 

[steveschuerger]

Looks like an encrusting sponge. It should be ok to scrub it off If that’s the case

 

[Willbiker]

Thanks Steve. If I scrub it with a toothbrush, it gives off quite a cloud of what looks like a chemical defence. Is this normal sponge behaviour ?