Welcome to the club Ronan! The first step is to clearly identify the "grass". Cut the top off of a clear plastic bottle (or use a good drinking glass from the kitchen) with smooth sides and put some of your grass in it with some tap water, then take your pictures. It helps to have a whitish background too. That will help those of us who might know what your dealing with. Then you'll likely get a herbicide suggestion. Sometimes a photo of the plant spread out on a white piece of paper can be helpful too. Does it smell if you break up a bunch of the stuff with your fingers? If so, smells like what?

I'm betting that is some kind of algae, but I'm not that good at ID'ing aquatic plantlife...yet.

Get some better pics back to the forum and we can get this sorted out pretty quick I'd think! Nice pond setting by the way.

Fish on!,
Noel