Prepping to pull out and replace some drywall (due to, among other things, what we believe to be an open leak to the outside) and found this behind the stove. the wall looks kind of blotchy on the right but near the bottom it looks like mold. should I be getting a professional in here? Thanks
It's mold. It likely poses no health risk, maybe unless you turn it into dust by sanding it and breathing it in, but you're not going to do that since you'll remove that drywall.
Drywall will get moldy if it stays humid for a long time. This can happen if there's a water leak on the other side, or a defect in the insulation which creates a cold spot, which will attract condensation. Quite often it will be hidden behind furniture, because that blocks airflow and prevents it from drying.
It's possible to bleach the mold, but that won't fix the humidity problem so it will come back. In this case mold is an indication that tells you to fix the root cause, which is the leak/humidity problem. This must be done before something else, like wood, rots and causes much worse problems.
For example: tenants complain about a moldy patch below the window that keeps coming back even though they clean it, and ask if they can put "anti-moisture paint." I look on the outside of the window and find a busted acrylic caulk seal, letting the rain in.
Spray some bleach on it.
If it is mold it will die.
If it is wet/water damage it will not go away.
You might want to consider a professional, to fix the leak and replace the dry wall.
You can get discoloration such as that due to water soaking through the wall and bringing assorted minerals with it. It can be difficult to distinguish this from mildew.
As Ruskes suggests, spraying it with some bleach will help sort things out, as bleach will usually cause mildew to fade, but will not often affect the coloration of mineral deposits.
