"Altmusic" or "Alt Music" is an abbreviation for the phrase "alternative music".

The abbreviation is commonly used when people are too lazy to actually spell out the whole phrase "alternative music" or hit a character limit. This abbreviation cost 9 less characters.

Altmusic may be shortened to 4 characters by typing "alt" followed by a music emoji but this isn't common since a vast majority of people that listen to alternative music are hipsters and think emojis are "too mainstream"