I planted sunflower seeds in a tray and it did grow beautifully,but I think I did put too much water in it and it has some parts with a white stuff .Anyone would know what it is this?Is it bad?What can I do to save it?

It's hard to say, but you might be describing mold. There are a lot of things you can do to discourage mold growth on your plant roots.

The simplest is to make sure you have good air movement around the roots. You can encourage air movement by planting less densely, and by growing in an environment with moving air. A ceiling fan, a table fan, or even a draft can fill the bill. I sometimes use a mister (spray bottle) to water between plants because it pushes as much air as water and helps to make sure the roots are getting to breathe.

If you overwater that may be the easiest way to prevent air flow. One thing I like to do when I'm growing in trays with soil is to place several wine corks around the surface of my solid tray under a tray with drainage holes. That creates a space for air flow under my plantings, and if I overwater there is a large space for that water to flow into instead of drowning my plants. The cork is good because it is naturally mold resistant - good for the wine, and good for your plants! (If you don't have dozens of wine corks sitting around, ask at a wine store, especially one that conducts tastings. I asked at a store near me and was immediately handed a sack of hundreds of corks.)

I know a lot of people use a chemical, I think it's grapefruit seed extract, to discourage mold. I haven't tried it, but I know people have had very good luck with it.

Thanks Frances,I'll try the wine corks.Maybe for the next one,I think that this one is a kind of dead.I did not water it for 2 days because I was waiting to dry off and than I'd water it,but when I got home from work today,it was a little down,kind of dead.Maybe I'll have some to eat,but I don't know yet.I'll check it up and will start another tray with corks.Great idea,thanks a lot.

Another thing,maybe it is the air like you said, it is near my wheatgrass,in a indirected sunlight in the kitchen.Maybe if I plant the other one near the window will help.