hard to tell exactly but my best guess is BGA (blue green algae) Here's some info I've found on it and most all forms of algae via a Google search as well as this forum.
If you can list your tank parameters then it'll help even further diagnose and effect a cure.
General description: It is green in color, slimmy and smelly. It doesnt really belongs to the algae family but is rather a type of cynobacteria. BGA spread very fast and it doesnt take long for them to conquer the whole tank.
1. Insufficient nitrate
2. Poor water circulation
1. Dose nitrate directly into the tank
2. Reduce lighting hours and if possible, switch off all lighting (black out)
3. Treat with Seachem excel - Fill a syringe with excel and then apply directly at the affected area (warning, some plant will melt in close contact with excel )
4. If all of the above not working, time for anti-biotic (Note you need to seed your filter after antibiotic regime)