Multipurpose Silicone Sealant for fake rocks?

[Catatafish]

Hi I was just wondering if its ok to use multipurpose silicone sealant to seal fake rocks after the grout?

 

[slantedii]

Mod podge is what they suggest to use, i even bought some yesterday tho i havent had a chance to use it yet. They say for high humidity environments you should use an acrylic sealer over mod podge. But the guy on lizard landscapes apparently uses mod podge too.