Tubeless sealant "seeping" through tires?

[makoti]

I don't really know what to call this, but after a ride and the bike sitting for days, I will notice what looks like dirt on the tires, but isn't. I'm wondering if the sealant is seeping through the casing? It isn't wet, but there will be more spots in the morning than were there the night before.
The tires have a lot of miles on them, they aren't new.
What am I looking at? Is this a good sign that the tires are ready to be replaced?
Conti 5K/Orange seal regular