Yeast Starter: Overflow - How Much Lost?

All-
Made a yeast starter that was 2.5L in a 3L flask on a plate and covered with foil. Belgian Aredennes yeast for a Tripel

Woke this morning and it had foamed over and there was a pile a milky, creamy stuff (I am assuming yeast) on the table.

I am wondering -

[1] Is this likely yeast?
[2] If the pile was about the size of a small side plate ... how bad is this? Can I still use the starter? Do I need to supplement? Same strain?

Ugggh.

It sure sounds like it's the yeast that has overflowed.
But rather than your second question, the better question is, how much yeast did it spill compared to what is left in the flask?

Usually, even with top cropping yeasts that overflow the starter flask (or fermenter), the vast majority of yeast is still inside.
What does yours look like, is there a thick layer of similar "milky, creamy stuff" in the bottom of the flask? If so go ahead and use it as normal, you probably only lost a small percentage.

However, in the unlikely event that most of the yeast escaped, repitch what remains into another starter (with more head room this time) so the cell count can grow back up.

Well, it seemed like it was a bit and there was less in the bottom of the starter 3L flask than usual... And I was leaving for a wedding for 4 days with no way to grab more yeast or re-do starter.

So I added the 3522 ardennes yeast from the starter AND the harvest trappist high gravity yeast from 2 months ago AND the harvest Wit yeast from last month. All in the belgian zone.

Guess I was willing to experiment. It is fermenting quite well now and I am on the way out.

Thanks for this post! I am currently sitting here watching my starter flask overflow (White labs American Hefeweizen) and I am concerned about how much is escaping. There is a decent amount of slurry building up outside, but I have what seems like a decent amount still inside. Also, the wort is still really cloudy so there is probably a decent amount still in suspension. I wonder though, if since all of the top yeast is coming out am I losing the best yeast that is in there? My only option is to pitch this starter, so I guess this will be a learning experience.