I think you used too much this time. A little goes a long way.
When using green stuff, the tips I've found the most useful are always wet your tool, if the is fingers a blade or a sculptor, always keep it wet. That way the green stuff doesn't stick and is much easier to work with.
Also the green stuff goes through 3 stages in the course of two hours. Super tacky, maluable and fine detail. In the first half hour just get it into place and with a very basic sculpt. If you just create the supporting structure there then Great.
Stage 2 make the general sculpt here as the green stuff will be less tacky and hold form well. The general detail, smooth it out and get it how you want the final thing to look. After this phase the green stuff will start to get harder to move.
Stage 3 the green stuff will hold its form really well and any fine details can be formed and even cut in it.
Tldr Be patient, don't rush. And keep a film of water on all your tools at all times.