It is not a grass and whether it is a "weed" depends on if it is a plant growing where you don't want it, and apparently you do not so to you it is a "weed". Since many unwanted plants grow in soils conditions that the prefered plants won't or can't I would start with a good, reliable soil test from a good, reliable laboratory. I would also dig in and look at that soil with these simple soil tests which will help guide you to correct the soil problems that exist and allow this plant to grow where it does;
1) Structure. From that soil sample put enough of the rest to make a 4 inch level in a clear 1 quart jar, with a tight fitting lid. Fill that jar with water and replace the lid, tightly. Shake the jar vigorously and then let it stand for 24 hours. Your soil will settle out according to soil particle size and weight. A good loam will have about 1-3/4 inch (about 45%) of sand on the bottom. about 1 inch (about 25%) of silt next, about 1 inch (25%) of clay above that, and about 1/4 inch (about 5%) of organic matter on the top.

2) Drainage. Dig a hole 1 foot square and 1 foot deep and fill that with water. After that water drains away refill the hole with more water and time how long it takes that to drain away. Anything less than 2 hours and your soil drains too quickly and needs more organic matter to slow that drainage down. Anything over 6 hours and the soil drains too slowly and needs lots of organic matter to speed it up.

3) Tilth. Take a handful of your slightly damp soil and squeeze it tightly. When the pressure is released the soil should hold together in that clump, but when poked with a finger that clump should fall apart.

4) Smell. What does your soil smell like? A pleasant, rich earthy odor? Putrid, offensive, repugnant odor? The more organic matter in your soil the more active the soil bacteria will be and the nicer you soil will smell.

5) Life. How many earthworms per shovel full were there? 5 or more indicates a pretty healthy soil. Fewer than 5, according to the Natural Resources Conservation Service, indicates a soil that is not healthy.

Thanks for the soil tips. I haven't had a soil analysis but I believe that it is on the sandy side. I live below the San Gabriel Mountains and the soil has some degree of decomposed granite. I do know that my soil drains quickly (less than 2 hours for the size of hole you specified). Thanks for the tip(s).

I'd like to see more pictures if possible. It could well be a grass from what is visible in the photo. It looks a lot like the semi dried, tangled stems of 'couch grass'(Agropyrum repens) which is a nuisance in UK gardens. There is probably a grass filling a similar niche in your area.