Hi, my mollie has also just come down with something similar. It has white patches, stringy like on it's back and sides. Noticed it yesterday, it came on in hours. My water has high nitrates. I changed 20% which reduced the nitrates and added ick treatment. He seems a lot more lively today.
I introduced a new fish on wednesday (platty) which was dead thursday morning. Then my mollie gets this. It's the second time I've had an outbreak in 5 weeks, the last time was after introducing fish from the same shop! I lost a mollie and 2 platties then.
Watch for your fish flicking themselves of rocks and other things, that usually an early sign something is irratating them.
Bob