It “looks” healthy from afar but hear me out please.

I recently moved my plant to the office and I noticed some of the leaves were turning a little yellowish at the tips. I thought it might be because less sun (I sit across from a big window so there’s still some light) but googling said “more light” causes yellow leaves.

I decided to move some rocks aside and noticed a black spot on the stem. Rot?? I’m not sure. It looks mushy.

I use the same filtered water that I drink to water it. I typically fill it up to about a half inch below the top of the rocks and wait for it to get about a half inch towards the bottom before filling it up again. I also noticed green (algae?) growing around the glass where water would be but I figured it was harmless cuz it has been there for over at least a year and hasn’t had any issues thus far.

If someone knows what’s wrong and what to do to fix it, please help. I love my plant so much. :((