Few months ago I saw this lichen on the wood.. Still it grows, but turn more brown at this time.
+1we need a full tank shot though!
It is a fungus rather than a lichen (symbiotic association between a fungus and an algae). These corticoid fungi are difficult to ID, but it looks like it might be a species of Aleurodiscus.I have similar thing on wood in my pet shop tank, but I thought it was some kind of mushrooms.
The filter may have mulmed up to that degree because the wood still had bark etc. which has supplied more organic compounds to the tank water. You can either put a pre-filter on the intake (I have fairly large sponges on all my tanks), or you can clean the internal media more frequently.That filter is disgusting by the way