1. Get that skimmer running asap, if it already isn't.
2. Suck as much of it out as you can. Airline tubing works good for this. If you have a python, you can use it "backwards" by putting some pantyhose over the large end and put the small end in the tank to suck up the crud. When you fill your bucket with water, dump it back in the sump so your return will pick it up, and repeat.
3. Do frequent water changes until you get things under control.
4. See #3.