The Rusalka is a rare corrosive foe, found principally in the Havensdale Bridge scenario. As with most monsters that incur a permanent debuff, avoiding taking damage from them is very advisable. Rusalka also leave corrosive puddles instead of blood pools. There are a number of ways to avoid these blood pools; using the WEYTWUT, WONAFYT, or PISORF glyphs to move the Rusalka before you kill it is one option. Another option is to carve an alternate path with ENDISWAL. Finally, you could use LEMMISI or BLUDTUPOWA to reveal a tile on the other side of the puddle, allowing you to "jump" over it. Curing corrosion is quite difficult, requiring a Burn Salve or a deity's boon.