Answered by u/the_stoned_ape:
That is what they call Yellow Slime Mold. Here is some more info and how to clean it.
Answered by u/zryan3564:
It can only be two things: Mold, which if it is, then it needs to go because of health reasons. It also could be expanding insulating foam. Either way, it it would be easier to see what it is if it's taken off. If the source of it is coming from a crack in the window sill, it's probably the foam. However, the foam doesn't melt like that unless gas or another chemical is put on it.
[If] you have something gross in the shower, you don't keep it there until you know what it is. You get it out of there and worry about it later.