1. They usually unscrew. Normal right hand thread.
2. Looks as if its quite heavily lime-scaled, which may make it difficult to remove.
3. Try (where feasible, taking care to protect decorated surfaces / carpets etc.):
3.1 Pouring very hot water over the shroud.
3.2 Using lime-scale remover such as Kilrock. Relatively agressive, so great care needed. Perhaps repeated applications with an old paint brush / tooth brush.
3.3 As CBW says, protect surface with cloth, or leather, or rubber and use a pair of grips.
3.4 Use a "Boa" rubber wrench.
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