About 10 days ago my large green leather toadstool stopped extending and a good sized patch of GSP next to it closed up. They both remained closed for about 5 days until they slowly started showing again. Nothing new was recently added to the tank and after two days of not showing I did a regular water change. Now that the leather has begun extending polyps again I am seeing the string like brown substance on it and some on the GSP. I have seen this before on some xenia in my tank but it comes and goes and doesn't seem to impact anything. Any ideas of what this is based on the picture?