Wow...I think I can help on this one.

1. I'm not there, but it does "look" like there was at least a little seepage from the upper radiator hose/clamp area. When that happens it almost looks like "battery terminal corrosion" & the outside of your thermostat looks that way to me in your picture. Antifreeze has corrosion inhibitors (anti-rust stuff) in it & a lot of those are white solids. When the water (& ethylene glycol) evaporate, the white solids are left. Just like if you dried out a glass of salt water & the salt is left in the glass. Corrosion inhibitors are salts too.

2. Seeing all that junk INSIDE the thermostat housing concerns me more. This "could" be "dried up anti-freeze corrosion inhibitor salts". Was your cooling system really low? I'm more inclined to agree with BlueRacer69. Did you use water out of your garden hose? "Hard Water" aka tap water & the worst is well water has a bunch of salts in it & they are not the kind of salts you want in your cooling system. Use distilled water or de-ionized water (about $1/gallon at Walgreens or grocery store).

3. I strongly suggest you flush your cooling system.
a. drain all coolant
b. fill with Prestone or other flush + distilled/de-ionized water. Make SURE you're outside temperature never drops below 32F while doing this since you won't have any "anti-freeze" protection.
c. drive the car for a whole day with the flush in it & make sure the engine gets fully warmed up & turn the heater on too. I would suggest you leave out the thermostat when doing this.
d. drain the water/flush
e. fill with just distilled/de-ionized water & drive at least 10-15 miles.
f. drain this water too.
g. fill your cooling system with fresh anti-freeze + distilled/deionized water (after installing your new thermostat)
h. for coolant, decent grade Peak Long Life (actually green Dex Cool) or Prestone yellow stuff works just fine. Use a decent brand & don't mix coolants of different types/brands.

4. If you want, you can check for "electrolysis" by searching online on U-tube or something like that. Basically, you will use an electric meter & measure current (or resistance? can't remember) with one meter electrode inside the coolant (radiator cap off) & the other end on the negative of the battery. I'm really fuzzy on this, so check what I'm telling you here. Electrolysis basically happens when stray voltage grounds itself THROUGH your coolant. I think the fix is to add an extra ground wire and/or clean your main battery/engine ground. Again, double check me on the electrolysis stuff.

Hope this helps!