Sounds like BGA.
See this extract from gf225's
Pinned article on Algae
Cyanobacteria (BGA)
Strictly speaking this is not a species of algae but a form of bacteria. It does, however, act in a similar fashion to algae and can be prevented using similar principles.
I am no expert on BGA (or algae for that matter) but I can share my experiences with it in the hope that it may help you.
Also known as Slime Algae, due to its physical nature, it covers the substrate and plants with a blue/green slime like substance. Untreated this will spread very quickly smothering everything, substrate, plants and décor. It out competes the plants for nitrogen and prevents photosynthesis due to it blocking the plants from the light. It is also toxic to fish and nothing will eat it. In short it is nasty stuff – it even has an unpleasant smell. This is one method of confirming it is BGA.
I understand it originates in the substrate, this is certainly true in the couple of cases I have had. I am unsure as to what the true cause of its manifestation but I think stagnant water helps it and the anaerobic bacteria associated with it. Low nitrates and natural light are often associated with its spread too.
Prevention
As with “normal” algae, prevention is better than cure. These steps should help.
1. Set-up your tank according to the above principles – in particular keep nitrates above 10ppm.
2. Avoid as much natural light as possible.
3. Try to keep the substrate from going too anaerobic. Some anaerobic areas are necessary and are found in nature. Poke the substrate occasionally, particularly if you have sand. Substrate heating can also be helpful in my experience. Hopefully with lots of root growth the oxygen produced will prevent too much anaerobic build-up.
4. If you see a slight BGA build-up between the substrate and glass (this is normally the first sign) then disrupt is by using a credit card. Slide it between the front glass and substrate. This should stop it from spreading to the substrate surface.
Treatment
There are two main methods to treating BGA. The first is a blackout. Cover the tank completely; do not let any light in at all. Wait for 3 days, no peeking or feeding the fish, then uncover, hopefully the BGA will have disappeared. I have not personally used this method but I am assured it works (except for Houndour’s case – sorry Sam).
The second is by using antibiotics. Remember that BGA is bacteria. Ethyromycine (trade name - Maracyn) is commonly used to rid BGA. It is also highly effective and is apparently fish and plant safe. The whole tank is normally treated via mixing the drug with the tank water. Dosage - 200mg per 10 US Gallon every day for five consecutive days.
I have had good results with Maracyn using another method. I recently witnessed a little BGA build-up between my substrate and front glass. I split a 400mg Maracyn tablet in half, then one half I broke into 8 small pieces. I placed each piece in the infected areas using tweezers. A couple of days later the BGA had completely disappeared. Now I slide a credit card between the glass and substrate every week just to ensure it doesn’t reappear. So far so good (8 weeks on) and I still have 99 ½ tablets left!
, any suggestions? 
its a case of take your pick
