Question about crts in a RCA Projection TV ppr475t (Chassies model-Hitachi VP6X)

[painkiller64]

Hi, I have this projection tv im cleaning up and well I was looking at each crt inside it (blue,green,red) and I noticed that there is a dirty rectangle stained in the middle of each individual crt. Well I thought that maybe it was the growth in the coolant so i cracked one open, drained the coolant out, wiped down the crt and its still there. Im thinking this is image burn-in but why would it be shaped and colored like that?

Any info would be nice

Thanks, Wes