soil Salt cube on my soil

Earlier I found a chunk of salt (I think) just laying on the soil on one of my containers. Am I overfertilizing? I'm using Miracle Gro for plants, following their instructions (every 7-14 days, I usually wait 14 days or more). A few leaves have yellowed and fallen in the past few days, but not many (5 maybe) and the plant looks healthy in general I think.
 


This is what I found
 

Hard to tell but it kinda looks like perlite.

Edmick said:

Hard to tell but it kinda looks like perlite.

Click to expand...

 
I hadn't realized perlite was so soft. I decided to post when I managed to break it but now I took some more perlite and broke it the same way. I feel dumb now.

Also, the more you water as time goes on, the more the perlite seems to work it's way to the top from the first half inch or so of soil, since it's lightweight and floats. So it can appear as though you're seeing more and more for a bit, then the soil settles and it'll always just look the same.

Could also be that watering washes the dirt off the perlite so it looks like there is more than when you started.