Its not indestructible, and it is part of the campaign experience, were (possibly) you team up with the covies in order to destroy the flood threat. and it can be destroyed by most heavy weapons, as mentioned in the article this picture it attached to, such as cyclopes or locust, or scarabs.
Don't read halopedia, the information is mostly unreliable and made up.
orcle:
Scrarabs in outer-space
Protecting the freedom of the covenant race