Looks corroded; can't tell if it's from inside the pouch's plastic or if it's from outside due to defects or damage in the pouch plastic, but the actual pouches have mechanical deformation, wrinkling, etc. Such wrinkling/etc means the cell layers are no longer attached to each other correctly, and/or there are gaps in the pouch-layer interface, so that electrolyte could be pooling in those areas and not be available in others, or even that air has gotten into the pouch thru pouch damage or defects. (air could degrade the cell chemically, as well as mechanically).
Damage can happen at the cell factory, battery build, or while riding if the pack design isn't sufficient to secure the cells and prevent it.
If it's not corrosion but is instead liquid electrolyte from inside the cells migrating thru defects or damage in the pouch's foil layer, but there is no external pouch plastic layer covering damage, then it's probably a manufacturing defect of the actual cell itself.
If there is any smell (especially acetone-like), from the cells, then whatever damage has occured has penetrated thru the entire pouch covering allowing electrolyte out and air in.
Overcharging you can test for with a voltmeter by verifying the balance of the cells, and/or the individual cell voltages vs the charger voltage once full charge is reached, vs what the cells should be being charged to for their specific chemistry/etc.
If the pack is still new and/or all the other cells are still matched to each other in characteristics, and you can find cells that match those, I'd repair the pack. But I would also make sure to test the BMS itself to be sure it is working correctly, and that it did not allow a charging or discharging problem to occur with the failed cells--it's not likely, given the physical defects/damage apparent in the pics, but it has happened.
If the cells don't match each other in characteristics, then either they have degraded enough to be problematic already, or the pack was never built matched to start with; and it is probably going to be better, performance-wise, to replace the pack with a new well-matched-cell pack.
If you can't find similar cells to what you have, at least the same chemistry and approximate capacity and physical size, it may make repairing that pack a difficult option. Cells with more capacity would be ok, but if they have less then your pack is limited by those cells. They need to be the same chemistry so they will have the same charge / discharge safety shutoff limits and your BMS can protect them correctly.
(Note that matching is not equal cell voltages, it is identical cell characteristics, internal resistance, capacity, etc., and is unfortunately apparently a rare thing for pack builders to do (probably because of the significant extra labor, equipment, and materials costs), so it is something you'd have to specifically inquire about when buying a new pack, and be sure to get actual test data for your pack's cells to prove that they are matched, since the typical response we see from pack builders is "we make sure they're the same voltage" and sometimes "we make sure they are all less than (some resistance value)" or "we make sure they are all more than (some capacity value", none of which have anything to do with matching the cells....).
