what is this on my plants ?

Looks like BGA

I agree, it is blue green algae (actually cyanobacteria).

There are various ways to get rid of it (a search will reveal lots of threads on this topic); in brief, you will want to fix your water parameters for the long term fix, but a short term fix is to use erythromycin.

How is the flow in your tank? BGA likes dead spots. Does the tank get hit with direct sunlight at all?

algae

Will this cause the tank to stink ?

The cyano does stink, especially out of the water but I don't know if it will actually make the tank stink.

rick dale said:

Will this cause the tank to stink ?

Click to expand...

yes it can bc it's not actually algae it's a bacteria. It may smell like a swamp or fishy.

bacteria

That's exactly how it smells. What do I need to do ? This tank is only 3 months old. Why did I get it ?

Yes it is Blue Green Algae.
This is not a true algae, but a bacteria called Cyanobacteria.
Even if you decrease the photo period and use combative methods used to get rid of traditional algae, it most likely will not work. You most likely have this stuff because of excess nutrients. I had a ton of this stuff in my tank and it spreads like a wildfire. You can spot treat it with a syringe full of hydrogen peroxide. This will almost instantly kill it temporarily maybe permanently, but I would not count on it to disappear forever using hydrogen peroxide.
Just do some research about it on google.
BTW, high concentrations of hydrogen peroxide can and will kill your fish and maybe your aquatic plants so be careful. I used about 1ml or less per gallon of water.
Do some more research on hydrogen peroxide on this forum or google.
Good luck!