You could probably use find/change to add the space.
I'd try the followng GREP:
\.(?![\d\s[:punct:]]) which I believe will find the periods not followed by any type of whitespace, a digit, or other punctuation (you might not want to change those elsewhere in the document if your search scope is not narrow enough to limit it to the descriptions, and even then there might be cases that would apply).
Replace with $0<spacebar>
For clarity, <spacebar> indicates you should type the space.
Can a GREP search be done within all instance of alt text? These are XMP Descriptions that are within images, figures, and other graphics, not always in structure. The problem is not in text, but rather in tags for 508 accessibility.
That's a bit above my pay grade, I'm afraid. I don't think there's a way to access tags with GREP, at least not through the Find/Change interface, and I'm sorry that I may have mislead you.