Asteroids look like this

http://legacysurvey.org/viewer/?ra=244.7872&dec=7.3889&zoom=13&layer=decals-dr2

This pair of blue & green objects is probably an asteroid. Blue=g filter and Green=r filter are taken back to back with DECam, so moving objects show up as slightly offset blue-only and green-only objects.

Why their shapes are square instead of round?

You know what, that might not be an asteroid after all! It might be a glint or some strange scattered light. It certainly doesn’t look point-source-shaped!

Good catch!

Here’s a sample of an asteroid: http://legacysurvey.org/viewer?ra=125.9116&dec=17.7958&zoom=16&layer=sdssco

SDSS has a particular green-red-blue pattern from the way the filters were set. That’s probably a Main Belter. A faster moving NEO would show three distinct points in each color instead of the overlap here.