Leaves turning dark/purple and 'leathery'

Hello!
 
I am growing Jalapenos and Seranos in a greenhouse in Spain w/ 2x 300W LED lights. I am new to chilly growing.
 
However, those I grow from seed are not actually my problem.
 
We also bought a (mature) pepper plant from a store around the corner. First, we have no idea what pepper this is, I am guessing it's some sort of ornamental pepper. It already had red, green, yellow and "purple" peppers all at once although I never had a clue which ones are the ripe ones. (I am guessing the yellow ones?)
 
For some weeks, this plant (along with another mature Bell pepper plant we bought) did well.
 
Recently, the leaves started to get a dark purple/black and the leaves feel dry and 'leathery'. (Older leaves mostly, young leaves still feel somewhat ok). But the entire plant is in pretty bad shape. I put it now outside the greenhouse just to see what's happening.
 
I considered many things already, over-watering, under-watering, too close to LED, etc...
 
What could the problem be?
 
I also did a mistake previously misting the leaves with water, you see the plant has horrible calcium stains and just looks awful. Any help appreciated so i can avoid this happening to my Jalapenos and Seranos.
 
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The first picture is from some weeks back when the plant was still somewhat healthy.
 
Thanks!!
 
Edit: Mold? I mean when I rub the leaves, nothing comes off. Root rot?
 
I am just baffled since the plant was doing fine for weeks and grew like crazy in the greenhouse.
 
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