It's hard to say for sure. They are using puns (growing on me, meaning gaining my approval, vs. actually growing like mold). Also, SOLID has become a slang word for GOOD in the past couple of decades. Or, they might literally mean solid, as in, not liquid, etc.

It seems to be the "hard" meaning. The "actually" and "you are joking" show that it's usually not solid (maybe a liquid) and that they are surprised.