Fungus. May be brought on by decay of material underneath the mat. Had an issue with it this week on some Jap deep waters and was recommended to make a bath with Furan2. One capsule or packet in 2.5 gal of water. Keep temp constant and aerate. Soak for 1.5 hours, rinse and return to tank. We'll see by tonight how well it worked...
Info is second hand so if anyone has tweaks for dosage/procedure, feel free...