Recently moved into first house, and have noticed that over the last week there has been a grey residue/liquid on the bottom of my last step outside of the front porch. This is an older home so I was a bit confused initially, we've had spotty rain over the last week but it's been dry over the last 24 hours and I noticed it was wet once again this morning. Upon further inspection I've noticed that the same liquid/coloring is in the mulch/planter touching the right corner of this walkway.

Hoping that it is due to old mulch/mold issue rather than a sewage/water line problem. Any clue what it could be?