Toilets, Sinks, Showers, Tubs and Disposals - Upstairs tub drain broken and stuck

The tub in our upstairs addition is about 15 years old. We recently noticed water dripping through the ceiling beneath the bathroom. I cut a square in the sheetrock and found the fitting that goes from the tub drain to the main drain was eroded (even though we don't use caustic drain openers and the drain has worked perfectly for years). The fitting is made of black plastic, not metal. I can reach it easily from below and have bought a drain wrench to fit the twist-drop drain in the tub. Here's the problem: The tub enclosure is fiberglas and the contractor coated the drain threads with some kind of clear plastic goop that has hardened. I'm worried that if I try to force the drain wrench, I'll destroy the bottom of the tub enclosure. Any ideas how to get the drain loose? (By the way, my daughter's boyfriend came up with a good idea of how to patch the hole in the ceiling: use a vent cover. That way it has permanent access and I don't have to refinish and repaint the ceiling)

Thanks, Bob.

I have no idea what the clear stuff might be. If all else fails, you may have to take a hacksaw and cut between the drainline and the tub drain and then replace the tub drain (and maybe the whole piping if you cannot then remove the threaded part with the goop on it.) Good luck.
Try chipping the hard goop off with a chisel. It may crack right off without damaging the tub.