Brown Sludge

I recently added some wild caught minnos to cycle a small tank.  I
decorated it with small hand made ceramic sculptures.  The sculptures
are now growing a reddish-brown slime.  I'm assuming this is from the
fish, or is it?  The fish seem pretty interested in the sludge, Im
wondering if this is going to be harmful after I remove the minnows
and add tropicals.
-Ryan