I got my new fish yesterday, i got 5 molly's, 2 catfish, and 2 shrimp.
they were all in the same tank before hand.

i left them over night and woke up in the morning to find one of my molly fish had white marks on its scales. i thought nothing of it and just thought it might of been in a fight or something.

Then i came back a few hours later to feed them, and i noticed it was swimming erratically, it couldn't swim straight, at the moment i have him in my net,

Is there anything i can do?

Are the scales themselves becoming discolored? If so, he's probably on his way out : (
Is it a cottony growth? This is fungus and you need to isolate and treat him and monitor the tank for signs of outbreak.
It is like small white cysts? This is white spot or ich, he needs to be isolated and medicated immediately and the tank closely monitored for outbreak.
I would up the Temp to around 82-86 degrees to help make the water less hospitable for bacteria, parasites and fungus. This will also cut down on it spreading and help your current sick fish have a chance of fighting it off, whatever it is that ails him.
Do you have aquarium salt? Not table salt. This is something that's always good to have on hand for out breaks, births and wounds. It will help make the, just like the heat, less hospitable for the bacteria, viruses and parasites as well as help to kill it in the tank and boost the fishes immune system, as well asrelievingg stress.
Right now I'd isolate him, and up the tanks water. There's not much moe you can do :(
Maybe describe the white's texture a bit and we can get a better diagnosis.
More than likelly it's shock and water quality that is causing the reaction, how bigs your tank? If it's a 10 or 20 you probablly over loaded it too fast : ( Take a sample of your current tanks water to the petstore (and if the fish dies, bring him as well with the recipt) most will do a free water test. Definitlly do a water change and look into what chemicals may be needed to right any water issues that could be off.
Best of Luck!

you should never put that many fish in a tank at one time. They get stressed out when they are moved and release nitrogen into the water which will kill them. If you dumped the water they were transfered with into the water, it added even more nitrogen to the water. You can try to treat the water with a balance product or nitrogen remover but it probably won't help that fish. I have been told by experts to never put more than 3 fish at a time into a tank and never dump the water into the tank.

It sounds like ich to me. Go to the store and buy some ich medication for it ASAP. If not treated, it will kill it. You can also give it a salt bath. Raise the temperature of the water. Ich can't survive in temps around 85 degrees. It's contagious.

You can put it in a pan of water an sprinkle aquarium salt over it...(SERIOUSLY it works) Hope this helps

it might be ick. A nasty disease it can ether go well or go bad. just buy some at walmart called ick away. It works trust me.

put it in its own tank for a coupel of hours,till it fixes! it should be ok when u put it back>

put it in a 5 gallon hopital tank with aquarium salt and sponge filter good luck