Disease, please help

Hard to tell from the pics provided, but it looks like it could be the start of a fungal infection. Fairly common occurrence, get yourself some Pimafix & treat according to the instructions on the bottle. Just make sure to run an extra air pump with & air stone or an extra power head (if you have either one handy) if you are dosing with Pimafix (or Melafix) as they tend to deplete oxygen from the water.

For future reference try to keep both Pimafix & Melafix handy in addition to a good Ich treatment. If you have these three basic/common meds handy you can head off a lot of diseases that you might encounter before they become big problems.

Better yet, get yourself a 5g or 10g tank & use it to quarantine new fish before adding them to your main tank, it'll save you lots of grief. Even a 5g bucket from Home Depot or Lowe's would work as long as you throw in filter with some media from an establish tank & a small heater. You can't observe your fish as well in a 5g bucket as you could in a glass tank but still beats just adding fish from questionable sources into your main tank & then having to treat the WHOLE tank instead of just a few fish. Plus when you are done you can break down the bucket & use it for regular maintenance. Just a suggestion....

Platys, mollys, etc. are bad stock from chain stores. Overbred and overstressed.

Hi,
With all due respect to DirtyJob, do not waste your money and crucial time to treat your fish with Pimafix or Melafix: they simply don't work. You should check the disease called Sliminess of Fish or Slime disease if the white patch is not a cauliflower-like growth. See the article (and perhaps articles you will find on other sites following your search) http://www.fishchannel.com/fish-health/freshwater-conditions/slime-disease.aspx
A "must have" book on fish disease is Dr. Chris Andrews and Adrian Exell's "Manual of Fish Health" in paperback and hardcover.
Good luck