There are a few things to try and do:
1. What games does it do this with.
2. Have you updated all your drivers.
3 Check your computer temps with http://www.almico.com/speedfan.php
or you can use Nvidia System Monitor but you will have to check if it's compatible with your card. http://www.nvidia.com/object/system_monitor.html
Your idle temperature for your computer should be around 40C and when your gaming it should be from 50-70C.
4.Take a screenshot of the green moss like effect and upload it to here.
5 If possible try to see if your graphics card fan is spinning and also if your CPU fan is spinning. Don't put your hand near it to see as the fans can be fragile as can your fingers.
Also post the rest of your specs: Motherboard, CPU and RAM.
Hope I helped