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If I recall correctly, you had "cured" some concrete and added it to your tank as base rock. Do you think this may be caused by the leaching of something out of the concrete? However, if it were caused by the concrete leaching I would think it would have been absorbed through all the silicone not just part of it. I believe it is some kind of algae because it seems to be spreading from the light downward. But I am not sure, especially when you say it will not come off. Even if it is algae it can be very difficult to get cleaned off, if you buy a green nylon pad like the ones for washing dishes and start scrubbing (without soap) on the silicone it should come off but it takes quite a bit of effort sometimes. You may also want to look at you light cycle and reduce it and definitely be sure your make-up water is low in nutrients. Besides this I really don't know what else might help.
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