You may have a chemical reaction going on in the water tank.

Drain out some water till it is nice and clear.

Also manually open the pressure valve till the water comes out nice and clear.

Note that you should drain some water out of the tank at least once a year. This clears out sediment that collects in the bottom of the tank.

Also looking at your picture it looks like there is not only foam but mineral deposits which indicate it has been leaking here for a long time.
Remove the plastic pipe attached to the pressure valve and check that it is clear. Any leak from the valve should have gone down the plastic pipe.

Your pressure valve may need replacing if it is leaking.

If it does then I would also pull the scavenger rod to see what shape it is in.
It may also need replacing..
Also check the bottom of the tank above the flame.

It may make more sense to replace the tank if it needs bothe the above parts.