You can tell that’s never been changed

That's the return, which shows 2 things. Generally speaking HVAC returns don't have a filter there it's at the furnace. That apartment was probably not cleaned very well by previous residents & the owners have never had the ducts cleaned. Before everyone gets all huffy you might want to take a look at your own returns. I've seen plenty of filters in apartments & businesses where they were never changed.

That not mold. It's years of dust and dirt accumulation on the turning vanes because the return in mounted at the floor with a stamped grill. Vacuum everything and install a filter back grill with filter.

That will have you breathing dust for eons

@Patriot_MAGA2 And god knows what else

That’s not mold, it’s years of dust build up and this is the return air duct.

That’s absolutely disgusting. I’d take my daughter right out of there. However, that’s a return vent. The filters are usually right at the furnace in a drawer just before the blower.

I got black lung just looking at this.

There's nothing to change in there. That is the vent where the air is returned to the furnace (where the filter actually is) Should still clean it but it aint a big deal.

Also those 9x9 floor tiles probably loaded with asbestos

As an owner of Air duct cleaning business, he needs one. That’s the return. Here’s the difference when you clean it. Pictures are backwards by the way smh.