Hello @Xanny and welcome to the Community.
You want your forearm and wrist and hand to be relaxed but your finger tips need to maintain grip. If the pick is slipping then try one or more of these: grips with a textured surface; holding the pick in a little so less of the tip protrudes from your grip, changing the angle of strum (is your guitar body upright, are you sitting upright, is your guitar neck angled marginally upwards and away from you?).
Hope that helps.
Cheers 
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