hi is your tank in direct sunlight ? do you perform regular water changes ? is your tank overstocked and do you overfeed the fish ? sorry about questions but just trying to establish some facts first,it sounds like a cyanobacteria bloom which may disappear on it's own if you remove the weed and as much of the snails as you can,the best thing is to test the water and the easiest way is to use test strips as it's mess free and takes seconds to do, you could have a nitrate problem,if so you need to do a 50% water change now and then a weekly 25% thereafter,you most probably also have high phosphate levels,if you have an external filter then put some rowaphos in it and this adsorbs the phosphate,your lights could be on for too long as well,try cutting down to 6-8 hours,if you have live plants this won't hurt too much for a short while,try all of these things in combination,cutting back a bit on the amount you feed can make a difference as well,sorry it's a bit long winded but can be a variety of things and not just one or two,good luck Paul