the chain on my stihl ms 250 won't stop spinning when the engine is at idle, even with the chain brake off. is that dangerous?

Yes. This is dangerous and indicates an incorrect idle speed adjustment. The chain must be stationary at idle.

Cause: Engine idle speed is set too high.

Fix: Adjust the idle speed screw (marked 'LA' on the carburetor).

1. Start the engine and let it warm up for several minutes.
2. With the chain brake OFF, check if the chain is moving. If it is, proceed.
3. Locate the idle speed screw (LA). Using a small screwdriver, turn it counterclockwise in small increments (1/8 turn at a time).
4. Wait a few seconds after each adjustment. Stop when the chain stops moving.
5. The engine should now idle steadily without stalling.

If that didn't work, the clutch may be worn or gummed up with sawdust, causing it to drag. This requires disassembly and cleaning by a qualified technician.

Heads up: Never operate the saw with a rotating chain at idle. It violates basic safety protocol. If you are not confident making this adjustment, take the saw to a STIHL servicing dealer.