1.  Timothy Hopkins.  1932.  John Hopkins of Cambridge, Massachusetts, 1634, and Some of His Descendants. Stanford Univ. Press, Stanford, CA.  On pp. 305-306:

In 1851 he removed to California, arriving on the ship "Columbus," and was a merchant at Sacramento.  His health failing, he became a farmer and stock raiser, and later was a capitalist.  A man of high character and lovable nature.

So… he was too ill to be a merchant, but well enough to be a farmer?  Methinks Moses turned to farming because he failed in business — our author does show many signs of being a crushing snob with regards to his adopted family (by adoption, Moses was Timothy's uncle).

2.  1840 Census Index/Images (online at Ancestry.com; Image #53-54 of 68; extracted by Diana Gale Matthiesen):

1840

NY

Niagara Co.

Lockport Village

pp. 92A-92B

Henry K. Hopkins

000 021 - 001 010

0000001

These data indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
2 males	20-29	1810-1820	= Moses (b. 1817) = Mark (b. 1813)
1 male	30-39	1800-1810	= Henry K. (b. 1807)
1 female	10-14	1825-1830	= ?
1 female	20-29	1810-1820	= Juliette (b. 1815)
1	persons engaged in learned professions and engineers

Henry was an attorney, and his brother, Mark, studied law under him, so Mark is undoubtedly one of the younger males.  The other male is probable their younger brother, Moses, who was also not known to be married at this time.  Both of their parents were deceased by 1837, so we do not find the unmarried children in their parents' household.  The younger female is too old to be Henry & Juliette's daughter, and Henry was not known to have had a sister, so perhaps she is kin of Juliette's — or an unknown wife of Moses.

3.  1850 Census Index/Images (online at Ancestry.com; Image #162 of 545):  New York City (Ward 15), New York Co., NY, p. 81A, 683/1112, enumerated 24 Sep 1850, official enumeration date 1 Jun 1850 (extracted by Diana Gale Matthiesen):

William Sherwood	60	M		Merchant		New York
Sarah	52	F				"
Harriet	29	F				"
L S Crittenden	20	M				"
Moses Hopkins	26	M				"
Martha Gilber	12	F				"
Mary Keley	35	F				Ireland
Ellen	23	F				"
Bridget	20	F				"

Moses should be 33, but he undoubtedly was not the informant for this record, so a mistake is not surprising.  He is the only Moses HOPKINS in New York (none in California).

4.  1860

5.  1870 Census Index/Images (online at Genealogy.com, Image #4 of 20 — indexed "More Hopkins"; online at Ancestry.com, Image #4 of 20):  Nicolaus P.O., Nicolaus Twp., Placer Co., CA, Roll 92 (Book 1), p. 107B, PN 4, 37/33, no enumeration date given, official enumeration date 1 Jun 1870 (extracted by Diana Gale Matthiesen):

Hopkins Mose

48

M

W

Farmer

20,000

10,000

Missouri

Moses should be 53 and born in NY.  The person above Moses, also living alone, was born in New York.  Did the enumerator mix them up?  Despite the errors, considering his wealth, his living alone, and the fact that he should be here, I believe this is our subject.

6.  1880 United States Census and National Index (LDS Family History Resource File: CD-ROM Library):

Census Place: Nicolaus, Sutter, California
Source: FHL Film 1254084; NARA Film T9-0084; Pg 410D
	Relation	Sex	Marr	Race	Age	Birthplace	Occupation	Fa	Mo
Moses HOPKINS	Self	M	S	W	63	NY	Farmer	MA	MA

He is the only Moses HOPKINS in California and the only Moses HOPKINS in the entire census who was born in NY.

7.  1890 Census:  the 1890 Census Population Schedules were destroyed.

8.  Anon.  9 Feb 1892.  "Moses Hopkins's Will." Los Angeles Times (Los Angeles, CA), p. 1:

Moses Hopkins's Will. SAN FRANCISCO, Feb. 8.—The will of the late Moses Hopkins will be filed for proate in San Mateo county tomorrow.  He left an estate valued at $4,000,000, three-fourths of which goes to his widow and the rest is to be divided among relatives.  He inherited his wealth from his brother, the late Mark Hopkins.

As far as I know, none of Mark HOPKINS's relatives, except his widow, inherited anything from his estate.  There's no reason to believe Moses's wealth was not accumulated on his own, especially as the 1870 census shows him well off even 22 years earlier (before Mark's death in 1872).