Scum on plant and on rocks

Question
hi, I have a saltwater tropical tank, about 22lt, I currently have 3 silver danios (freshwater fish surviving quite well in saltwater) and 1 sucking catfish (did have 2, but I think the one of them chased the other from the food all the time).

But I've noticed some strange colors on one of my plants and also quite a bit of scum on the rocks.

I have a filter running all the time, 400lt/hour.

I add an extra lt of water each week after evaporation, and do a 10lt water change every month, I've only had the tropical setup for about 2 or 3 months. See website of my pictures;
http://users.tpg.com.au/kellieaw/scum_on_rocks.JPGhttp://users.tpg.com.au/kellie...

What's the cause?  

Answer
Hi Matt;

Well, I don't usually take saltwater questions, but it looks like algae to me. Just scrape it or wipe it off and make more frequent water changes to reduce the organic wastes it needs to grow on. Alga is a natural and very normal organism.

At Your Service;
Chris Robbins