It looks like dried droppings, and they can be very hard, like concrete. I also see some white clumps at the bases of some of the feathers and can't tell if that's just dried urates (the white part of the droppings) or if it may be lice eggs. This link has pictures of lice eggs for comparison:
https://the-chicken-chick.com/poultry-lice-and-mites-identification/
So if there is lice, you will need to treat for that.
If not, then you can soak and remove as much as you can, you may have to clip some feathers off to get it all removed. It may take more than one soaking. You can use small scissors to clip back feathers. There are many reasons for dirty vent feathers. Internal parasites can cause runny droppings. A fecal float test will rule those in or out, see if you have a vet that will run one for you, it's not terribly expensive if they will do it. There are also mail in options if you don't have a vet that will.
https://www.amazon.com/dp/B000J5SOZ...colid=27RHKHAM35GO&psc=1&ref_=lv_ov_lig_dp_it
Some hens don't push droppings out as hard as others, and it can result in dirty tail feathers like this. It can just be a factor of age or genetics, or it can sometimes be a sign of a reproductive problem making it harder for them to push droppings out. Do you know when she last laid? Stoppage of laying and a bloated abdomen, between the legs below the vent, are also symptoms of reproductive problems.
When they get this much buildup of droppings around the vent then flystrike can become a risk, and that is serious and can be fatal, so I would clean it up as much as possible, and clip feathers below the vent to keep it from getting bad again. Flystrike is caused by flies laying their eggs in the built up droppings and then maggots hatch and will actually feed on the live chicken.