Guck growing in paludarium - solutions?

[m_ms09]

I built a paludarium not long ago (like a terrarium, but with water on the bottom) -- it has a fogger that humidifies the air 3x a day for a couple hours each time, but there isn't enough water movement to prevent algal growth/etc. Right now there is a black guck (like a sludge) forming on the rocks on the bottom.

I'm growing orchids and ferns in the paludarium which are watered by the water in the bottom.

Question: What can I use to eliminate the guck? Will hydrogen peroxide kill the guck but spare my plants?