If $monster is an array and mysql_real_escape_string as the name implies, takes a string, you must pass it a string.
Since the function takes a $monster_name, perhaps you need to take that from the $monster array?
Something like (without any idea of your code, just as an example, do not copy and paste 🙂 ):
update_save($igralec_ime, 'gozd', $monster['ime']);
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solved Function expects array to be string, convert?