Identifying a glass frosting chemical
The most common glass-etching powders are solid ammonium bifluoride and solid potassium bifluoride. See for example the Material Safety Data Sheet for “EtchON Acid-Frost Glass Frosting Powder”, or this one. This is not a definite identification, but it fits your description, and those two are the most commonly available such chemicals.

Disposal is a bit tricky. Definitely don't put it down the drain! The first MSDS linked above says:


  Ecological Information:
  Noxious for the water inhabitants. Avoid disposal into rivers, sewer system and land penetration.


The second gives a hint at disposing of it:


  Disposal Instructions:
  All wastes must be handled in accordance with local, state and federal regulations. Material can be converted to a less hazardous material by weak reducing agents followed by neutralization.


So, contact your local waste management authorities, and ask them how to proceed. Do not attempt it neutralize it by yourself unless (i) you know what you are doing, and (ii) you have the appropriate hardware (fume hood, e.g.).