Okay, so there might be a bug here. I killed the demon the first time, & germinated the yommi seeds in the water brfore I investigated the pool. Now, all Guojo wants to talk about is growing the tree. How do I get him to talk about the pool & the cave? Or should I just start tackiling that on my own?

Maybe you skipped something. Use TipIt's quest guide if youre stuck and try to find out if you skipped anything.

I had the same problem, you need to be next to the pool, try to get water and it will turn muddy. swing your bullroarer there (next to the muddy pool) and he should talk to you about it. Good luck.