Be sure never to let coco dry out, not even a bit. The top layer should always show at least some level of moistness imo.
When it dries out too much ph swings way out due to a buildup of the nutes getting way too concentrated, make sure you are feeding/watering regularly enough. Coco needs a lot of
calmag normally, but as another poster stated many times it contains nitrogen also.
Yur leaves aren't overly dark green, and not clawing, so I'm not sure I would put my money on a nitrogen toxicity. Normally blotchy and rusty spots do indicate a possible calcium def. Get your watering/nute strength/schedule down and you should be fine.
Also make sure to give the medium a really good water through (with nutes and/or without, depending on run off ec/ppm) every couple of weeks or so, to get rid of excess salts.
Hopefully something helps! :)