Plant is indoor, but in place with a lot of light, Eastern exposure with buildings blocking only small part of it (maybe too much of it?).
Windows are covered by something that supposedly is UV screen.
Any idea what is going wrong? At this point I am considering throwing it away with soil and retrying, but at least I would learn what went wrong here.
And maybe plants/soil can be still rescued somehow?
The strange mosaic-like patterning puts me in mind of a virus. Can't say which one for sure, but if I'm right this means that the whole planter is as good as dead. Toss it in the garbage, get a new pot with new soil, and start from scratch.
While I've never seen them attack parsley personally (especially indoors), the damage looks just like damage from leafhoppers (e.g. variegated grape leafhoppers) or thrips. If you disturb the plants and see things fly off all over the place, those are likely leafhoppers. They tend to hide on the undersides of the leaves. I have seen variegated grape leafhoppers on a number of herbs (as well as strawberries, and of course grapes); so, I wouldn't be that surprised to see them on parsley (if it were outdoors; indoors, it would surprise me).
Thrips aren't always so visible as leafhoppers, but they can do similar damage on roses and things. My guess is it's thrips.
Whiteflies are another possibility.
Another consideration is scale insects and similar.
Anyway, I'm pretty sure it's a creature of some kind causing the issue.
Parsley is an annual that will overwinter. In the second year it will run to seed, (if you want to harvest for the third year) and run down. Indoors it is either, being harvested or mistreated. A second year it tends to wilt.
