Tsumasaki Ni Kourozu

From Echochi Scans:
Iwai Arata was recommended by his professor to work part-time under a rather eccentric mycologist, Murota. The mycologist seemed hard to approach at first. But slowly as he got used to Iwai’s presence, he became friendlier in his own way, showing Iwai a whole new world and giving him guidance with life. Though every day was fun and exciting, a sense of unease began to creep into Iwai’s heart