This one is a doozy...
Firstly, in Hebrew,
arel means 'imperfect', 'unclean' and 'uncircumcised'. Arella would therefore be feminising arel by tacking on an -la (in the same sense that one would feminise
Aviv or
Adam by tacking an 'A' to the end of the name), however arella doesn't exist as a word.
I've done some digging and according to a couple of sites I trust, Arella is the feminine form of Arel which does, in fact, mean 'angel'. I'm perplexed, though, because the (transliterated) Hebrew word for angel is
mal'ach.
So, on one hand, you're correct in that it means 'angel' or 'messenger', but on the other hand, it means 'unclean' and 'uncircumcised'.
I'll try to do some more digging.
Mashka