The form he's stuck in is a merman form. But it's not, like, a typical merman form.
He'd been trying to see if he could reshape his ecto while he was in human form, and he had! But he'd also gotten overenthusiastic in experimenting, added too many features unique to his ghost form, and now he can't shift into either form.
Human or ghost.
He's stuck.
He's stuck as a merman with his human features, his ghost-form eye color (it glows), his human form hair (tinted with glowing green-ish white), deep blue scales on his tail that fade into that seem glowing greenish white, and fully functional gills.
He also cannot use his ghost powers. He knows that they're there, but for some reason his current form will not allow their usage. Maybe...maybe he's using too much ghost energy to keep up the transformation?
Regardless, he got captured by some sleezeball and thrown into a traveling freakshow. Highly illegal, and he's slowly losing hope that he'll be able to escape, because he just. Can't. Shift. Back!
Then the sleezeball makes a stupid mistake; he puts the freakshow up in Metropolis.
Danny goes to sleep one day closer to a mental breakdown.
Danny wakes up to Superman floating in front of his tank, in the process of restraining Aquaman from quite literally murdering the sleezeball.