Which weed killer for artificial grass?

[Spartan luke]

Evening everyone. Can any one recommend a weed killer to use on my artificial grass. My garden was a weed pit so I had the artificial grass fitted. The transformation is amazing but one thing iv noticed over the last few months is lumps appear under the grass. Has anyone else had this problem with fake grass and as the title suggest I'm looking for a weed killer that doesn't destroy the artificial stuff or discolour it in any way. Here's a pic too

[Spartan luke]

They are weeds not moles. All the ground work was done prior to having the grass fitted but weeds grow back fast in my garden so I have to apply weed killer to keep the weeds at bay but like I said I don't know what to weed killer to use so it doesn't bleach or melt the artificial stuff. The artificial grass has small holes which the odd weed pops out from so I just need to keep in top off them. I'd rather pull a weed out every once in a while than cut it every couple of weeks