Yes, it has an exclamation point on it too. Talk to Pirou at the Gerudo Canyon Stable during the day and you will find that he is a major Rushroom Rush fan. So much so that he will give you a Diamond if you can find him 55 Rushrooms.
That's the entire point of this quest. After you have collected 55 Rushrooms, hand them over and a treasure chest will spawn behind the stable containing the promised Diamond.
In the future, you can give Pirou another 55 Rushrooms and he will buy them off you for twice what they usually sell for. If you usually have no use for Rushrooms, you can offload them onto him.