Purple Spikey thing

I have a rock of this stuff. Its more dark than yours, however they're probably similar species. I've kept the rock isolated from all other rocks, however it just grew onto the glass (barebottom tank). in some areas its almost as strong as epoxy. it inadvertently killed a hermit crab that was molting for a couple days, the carbs shell was touching the sponge and within a few days the spicules had grown onto the shell. Creepy but cool IMO

Exposure to air does nothing.

I also have a pink variety, same structure, however bright pink and a bit slower growing.

You could try using some small pliers to get it off (only thing that really worked for me)