Of course he's right. The white in the top right is far too bright and gives it the sort of shape grass doesn't deserve... Grass is flat (but higher than the ground), what you have there looks round.
Also put some figures in there, that'll help our brain to put things into perspective.
Also your brushes look more like tree leaves than grass blades... Again ptoing has it right, try /smaller/.
And then there's the arrangement and the background color... For the arrangement, you should try many small bushes; For the background color, try something grassier (but I suck at picking colors so don't ask me)
PS: I'm drunk so I might not write what I think or what I think might be wrong. Don't worry if you don't understand this post - it's not you, it's me. Kind regards, Jonas