jokunokun

I'm new to the hobby and have a new tank set up, about 2 months old. The water tests as healthy all around (ph is on the low side), although I don't have the stats here. Here are the details:

20 gallon high
100% flourite substrate
stock 15w lighting for now
no additional ferts or CO2

I've got some kind of reddish-brown film growing on one of my plants and a plastic fish cave decoration. I'd noticed it on the plant, some kind of rosette that seems to be growing well, and just noticed that the cave seemed to be getting darker and darker. I checked it last night and finally realized there was a film collecting in all of the nooks and crannies. I have not idea what it is, and no other problems to speak of in the tank. I'm going to pull it out tonight and give a good soapless scrub, but I don't want to ignore a potential problem. Any help would be appreciated. No pics at the moment, unfortunately. I'll try and come up with some later, although it'd be pretty hard to see.