What is this residue and how do I get rid of it?

[itstoohot]

The chlorine level is very high - waaayyyy over 15 ppm. How high should the Cl level stay to fight mustard algae and for how long?????The stabilizer level is 80.

With a CYA level of 80, you need to be at 20 ppm of CL to even reach shock level. See this thread - http://www.poolforum.com/pf2/showthread.php?t=365

We are finishing our fight against mustard algae and have been maintaining shock level for nearly a week now. You will need to do the same. Don't let it drop below 20 until the algae is gone. It will also help if you can vacuum to waste as the stuff dies and drops to the floor.