Whenever I travel to Heljarchen Hell with Serana'sreanimate corpse, it turns to ash piles. Now, I had two ash piles outside Heljarchen Hall that can not be clean up, and one of them active option doesn't work.

Use the console to delete them. I can't remember the command off hand but I'm sure it's in one of the wikis.

Do you mean "markfordelete"? It doesn't work. Now, I have a dead dragon's body outside my Lakeview Manor not cleaning up either -.-

Look for an ash pile deletion mod or make one. Those are "activators" and never go away on their own.
The best deletion mod will delete them based on a timer.
A properly made mode will affect ash piles and goo piles.

This problem has been around since Fallout 3 and the only fixes for it have come from the modding community, so far.

The console commands involve clicking on the ash pile, not always possible if fog is in the room, then tapping in Disable, then MarkForDelete.