As you suggest and note in your question, and it deserves repeating:
- Aluminum of any kind is generally not recommended for the dishwasher: it is likely to corrode / oxidize like your pot did.
- Keeping your moka pot clean and dry will also help reduce the formation of pocking and oxidation.
That said, while unsightly, it should not be harmful to use after going through the dishwasher. You should thoroughly wash by hand no matter if you polish or not; you should remove any leftover polish, aluminum oxide, dish detergent, or any other residue (also suggested by Ecnerwal in a comment on the question). Polishing is possible with acids or metal polishes, but will probably not provide more than temporary, cosmetic benefit. That is my experience when trying to "restore" accidentally-dishwashered aluminum: I washed it thoroughly, and I continue to use it in its tarnished state.
I couldn't find a really good reference about these recommendations; here's a discussion on Chowhound with some reasonable information; many more by searching "dishwasher aluminum" or similar. Some suggest strong chemicals and abrasive scouring pads; I personally wouldn't use these for fear of further damaging the pot.
My personal pick is to use a stainless steel moka pot, affording a little more margin for error on cleaning. That said, aluminum models heat quickly and certainly do look classic and classy.