Meaning:
“脱” means “to break away from,” and “粉” is short for “粉丝群,” which means “fan base.” Often used by fans of pop idols, this term means “stop being a fan of.” This usually happens when the pop idol one previously likes gets involved in a scandal or other negative things about them have occured.
Example:
A: 你喜欢的那个歌手最近要来我们市开演唱会，你不去捧场吗？
Nǐ xǐhuan de nàge gēshǒu zuìjìn yào lái wǒmen shì kāi yǎnchànghuì，nǐ búqù pěngchǎng ma？
The singer you like will come to our city to give a concert shortly. Won’t you want to buy a ticket?
B: 我已经脱粉了。
Wǒ yǐjīng tuōfěn le。
I am no longer his fan.