Center of leaf browning

I found a leaf that looked not like the others! It's off one of my weaker plants, but this is definitely abnormal.

I just want to know if it's a serious problem or just a fluke. Most of the other leaves look OK, however, a few have splotches of light almost white areas.

Here's the leaf with the browning towards the center. The little white things are flecks of perlite, I believe, not mites.

Looks like the early signs of a phosphorus deficiency. Just going off of what I saw from google pics, but out of the different deficiencies it looks the closest to me...