I am delighted that there are other people wondering about strange
and beautiful things growing on their plates, even when these things
might be contaminants... :->.
A good test to find out is colony-purifying your stock: if you lose
the weirdos, they probably were contaminants; if you don't, they
still could be contaminants that stick to your yeast cells. You
might try to manipulate off single cells and watch what grows out
to make sure.
P.S. Have you ever seen yeast colonies that stand straight up on a plate
and look like fingers? I occassionally get these, especially after
mutagenesis, and they were no contaminants; upon restreaking, they
gave rise to normal, boring, flat yeast colonies.
P.S.2. Of course, I do incubate my plates upside down, so that the
"finger-like" colonies are hanging, not standing when they grow...