Whole house flow through type humidifier mounted on evaporator coil cabinet wall. I opened the humidifier cover, evaporator coils exposed!
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Looks like the humidifier screen to me, not the evaporator coil.
Yes, that is an expanded metal element of the humidifier.
I usually see them in the return air side here.
Check mfgs. instructions.
I’ve installed similar “By-Pass Type” Humidifiers on the supply plenum in the past. The catacomb ceramic or paper screen filter looks like it’s pass due needing to be replaced. It probably could have been cleaned earlier on, but looks like it has passed that stage. Maintenance is critical to efficient, leak proof operation, and if allowed to get to this stage, is a possible algae/mold breeding problem. I recommend having it checked and cleaned by a pro HVAC Contractor.