Yes, Maryland is allowing construction companies to continue their work and has designated them as essentially businesses. To be sure, the interpretive guidance on Maryland's stay home order specifically states " Commercial and residential construction companies" are essential.
Obviously, this could change in a hurry. So, to stay on top of things, it'd be wise to keep an eye out for additional guidance from your local or state government. For help with that, this tracker should help: Can I do Construction Work In my State?