Alter self (PHB, pp. 211-212) includes the following on changing your appearance:
Change Appearance. You transform your appearance. You decide what you look like, including your height, weight, facial features, sound of your voice, hair length, coloration, and distinguishing characteristics, if any. You can make yourself appear as a member of another race, though none of your statistics change.
That it says "you decide what you look like" and that you are allowed to change "coloration" suggests that maybe this could work to include bioluminescence, but on the other hand, usually anything that is considered a source of light includes it radius (usually describing bright light and dim light), which alter self does not.
Contrast this with something like produce flame (PHB, p. 269):
The flame sheds bright light in a 10-foot radius and dim light for an additional 10 feet.
It sounds as though the bioluminescence you're after isn't necessarily to actually create any "real" light (since you said "casting superficial light, or dim light at best"), so a DM might allow it, but the fact that this is not mentioned suggests that this wouldn't be what the spell offers RAW.
As an alternative, if you know the light cantrip, you could cast that and work that into the "disguise"?
If it's a halo you're after, then as an aside, there is one way to get something that is unambiguously a halo, but it's not easy...
This is a legendary artifact that will only allow good aligned creatures to attune to it, but it does bestow the following benefit (DMG, p. 222):
Halo. Once you've read and studied the book, you gain a protective halo. This halo sheds bright light in a 10-foot radius and dim light for an additional 10 feet. You can dismiss or manifest the halo as a bonus action. While present, the halo gives you advantage on Charisma (Persuasion) checks made to interact with good creatures and Charisma (Intimidation) checks made to interact with evil creatures. In addition, fiends and undead within the halo's bright light make attack rolls against you with disadvantage.
This would make your disguise very believable, because your halo will be a real halo, but having access to such a powerful artifact probably makes this solution unviable... but, y'know, for completeness.