Lady Seolmundae is translated as grandmother Seolmundae in some cases but I don’t think the direct translation from Halmang in Jeju language makes sense. The reason of calling an old lady as Halmang(grandmother) is the way of a respect in Korean culture so English translation to grandmother sounds a bit awkward.
There are quite a few mythical stories about Lady Seolmudae.
Here is one of the most well-known myth of her.
She created Jeju island by carrying soils in her skirt and pouring into the sea. When she realised the centre of the island(which is Mt. Halla) was too high, she cut off the mountain peak and threw it away. Baengnokdam(a crater lake on the summit of Mt. Halla) was created and the thrown peak became Mt. Sanbang(the famous Oreum at the southern west side of Jeju). People say it is a co-incident that the circumference of the crater is about the size the perimeter of Mt. Sanbang whether you believe or not. While she carried the soils to create the island, some soils were spilt through the holes of her skirt and became Oreums(volcanic cones).