Your plant is presenting symptoms of a physiological disorder, oedema/ edema. Something I wrote that should help you avoid it in any foliage the plant pushes in the future:
Oedema (aka edema) is a physiological disorder that can affect all terrestrial plants. It occurs when the plant takes up more water than it can rid itself of via the process of transpiration. The word itself means 'swelling', which is usually the first symptom, and comes in the form of pale blisters or water-filled bumps on foliage. Under a variety of circumstances/cultural conditions, a plant's internal water pressure (turgidity) can become so high that some leaf cells rupture and leak their contents into inter-cellular spaces in leaf tissue, creating wet or weepy areas. Symptoms vary by plant, but as the malady progresses, areas of the leaf turn yellow, brown, brown with reddish overtones or even black, with older damage appearing as corky/ scaly/ ridged patches, or wart/gall-like bumpy growth. Symptoms are seen more frequently in plants that are fleshy, are usually more pronounced on the underside of leaves, and older/lower leaves are more likely to be affected than younger/upper leaves.
Oedema is most common in houseplants during the winter/early spring months, is driven primarily by excessive water retention in the soil, and can be intensified via several additional cultural influences. Cool temperatures, high humidity levels, low light conditions, or partial defoliation can individually or collectively act to intensify the problem, as can anything else that slows transpiration. Nutritional deficiencies of Ca and Mg are also known contributors to the malady.
Some things that can help you prevent oedema:
* Increase light levels and temperature
* Monitor water needs carefully – avoid over-watering. I'd heartily recommend a soil with drainage so sharp (fast) that when you to water to beyond the saturation point you needn't worry about prolonged periods of soil saturation wrecking root health/function. Your soil choice should be a key that unlocks the solutions to many potential problems.
* Avoid misting or getting water on foliage. It slows transpiration and increases turgidity.
* Water as soon as you get up in the AM. When stomata close in preparation for the dark cycle, turgidity builds. If you water early in the day, it gives the plant an opportunity to remove (for its own needs) some of the excess water in the soil.
* Put a fan in the room or otherwise increase air flow/circulation. Avoid over-crowding your plants.
I noticed your plant is potted in CHCs. Not all plants tolerate CHCs or coir well as a primary fraction of a grow medium. High levels of phenolic compounds known to be present in coir/ CHCs can profoundly affect root morphology, limiting length, number, and size of all orders of roots (1st, 2nd, 3rd, ....), and does interfere with cell division. Too, the chemistry of both products varies enough from peat and pine bark respectively that the coconut products shouldn't be thought of as direct substitutes for either peat or pine bark. In the case of coir, it's loft is significantly less than that of peat, so when media with large fractions or coir are in use, top watering can cause a notable amount of compaction. Most greenhouse/ nursery ops that include these products in their media limit their presence to 10% or less, and in the case of coir, most watering is from the bottom in ebb/flow systems.