Out of curiosity, why would you suggest the OP to use erYthromycin? It is an antibiotic used to treat gram positive bacteria. It will crash his system & just make things much worse.
...... can anyone hypothesize why this occurs? -- IMO primary research needs to be done. I too would be very interested in finding out why dats develop the "white spot" within the eye. I have seen dozens of dats that have this -- even in systems that are understocked, well maintained. Dats (indos) are in abundance but mysterious at the same time. We simply don't know much about them.