Toadstool problem

[Ewatson249]

Anyone know what this us on my toadstool. It appeared a few weeks ago so I used a clean knife to take it off. Unfortunately six days later it had come back even worse. I'm worried it may start contaminating my water so am thinking about removing it. Any ideas anyone?

 

[ccCapt]

That's dead/dying flesh. That leather doesn't look very happy.

 

[Ewatson249]

Yes that's what I thought although 2 mins after that photo all the polyps were fully extended and it had doubled in size looking very healthy apart from the obvious. I had only just put the lights on when I took it. At what point should I take it out?