Your soil mix looks ok, it might be a little too on the heavy side, but the plant looks over potted. There is too much soil for the root mass, it looks like its holding water too long, and leading to issue with your roots. A more porous better draining mix would help.
You can remove that leaf, it won't recover.
This looks like an older plant, from the look of the length of the stem and the number of old cataphyls. It should be more full.
I would also inspect the leaves very closely for spider mites.
On a plant with the naturally mottled color pattern as this Aglaonema, damage from mites might go unnoticed for a while.
Its a strange thing, but Aglaonema CAN be grown as emersed aquatics if you unpost the entire plant with soil and stick it in a pond, but they do NOT like wet feet when in a pot
